Optimizing nitrogen fertilizer amount for best performance and highest economic return of winter wheat under limited irrigation conditions.

Inappropriate water and fertilizer management can lead to unstable crop yields. Excessive fertilization can potentially cause soil degradation and nitrogen (N) leaching. The aim of this study was to explore the optimal N application rate on two wheat varieties with different nitrogen responding under limited water irrigation at three experimental sites in the Piedmont plain of the Taihang Mountains, China. A two-year field experiment was conducted to explore the effects of five N application rates (N0, N120, N180, N240, and N300) on winter wheat growth, leaf area index, aboveground biomass, grain yield, grain N accumulation, and net return. The results showed that N application rate significantly affected leaf area index, aboveground biomass, grain yield, and harvest index. Variety and variety × N rate interactions had a significant effect on few indicators. Compared with N0, N180 improved leaf area index, aboveground biomass, grain yield, and grain N accumulation. Compared with N240 and N300, N180 increased the harvest index and N harvest index, without significantly reducing grain yield or grain N accumulation, while enhancing a higher N use efficiency. Fertilizers applied in the ranges of 144.7-212.9 and 150.3-247.0 kg ha-1 resulted in the highest net return for the KN199 and JM585 varieties, respectively. Our study provides a sound theoretical basis for high-efficiency fertilizer utilization in sustainable winter wheat production in the Piedmont plains of the Taihang Mountains of China.

Introduction

Wheat (Triticum aestivum L.) plays a highly significant role in the nutrition of the Chinese population [1]. Further, the North China Plain (NCP) is one of the primary crop production areas in China, where typical winter wheat is planted in approximately 61% of the total arable land [2]. Winter wheat production in this area plays an important role in ensuring food security and avoiding energy crises in China [3]. However, to continuously maximize crop yield, farmers frequently use nitrogen fertilizers and irrigation excessively. Meanwhile, by and large, groundwater irrigation is the primary irrigation source used across farmlands in the region. Thus, as a result of the excessive exploitation of groundwater, decreasing water availability for agricultural production has gradually restricted the sustainable development of local agriculture [4-6]. Concomitantly, the excessive use of nitrogen fertilizers threatens the ecological safety of the area [7,8] and may increase greenhouse gas emissions [9]. Therefore, increasing winter wheat yield while reducing water and fertilizer inputs, maintaining sustainable crop production, and improving the agroecological environment have all been imperative to achieve the sustainable intensification of agricultural systems in the North China Plain [10].

Sustainable agricultural intensification needs to optimize genotype × environment × management interactions for each target environment [11]. Over the past decades, the development of wheat varieties has contributed greatly to yield increases [12]. Nitrogen absorption by winter wheat is reported significantly correlated with different efficiency varieties [13]. Thirty-nine superior wheat varieties were planted at five N fertilization levels ranging from 0 to 350 kg ha-1. The results showed that all interactions between variety and N-rate were highly significant for the total N absorption and N use efficiency [14]. Total N absorption by high-N efficiency cultivars was higher than that of low-N efficiency cultivars [15]. In the NCP, the range of N input is 117–455 kg ha-1, but only 35% of the applied N fertilizer is absorbed by winter wheat [16,17], which leads to massive N loss and subsequent negative environmental impacts [18]. Selecting high-efficiency wheat varieties and optimizing agronomic management practices to increase water/nitrogen use efficiency (NUE) is an effective strategy to increase crop yield and reduce associated environmental costs [19]. The most suitable N application rate varies with planting area and wheat variety. Mehrabi et al. [20] showed that there was no significant difference between 150 and 300 kg N ha-1 in Shiraz, Iran. The optimal N application rate is 150 kg N ha-1. In turn, Xin et al. [12] showed that 150–210 kg N ha-1 in winter wheat cultivated in the NCP resulted in higher productivity, higher NUE and lower N2O emissions.

Although previous studies have optimized fertilizer input to maximize crop yields or net return, few studies have conducted multiple experiments in the Piedmont plains of the Taihang Mountains to investigate the effects of varying N application rates on wheat varieties with differences in factors affecting sustainable agriculture, including N efficiency, wheat yield, N uptake, and net return. Thus, understanding and mastering the N absorption-related in wheat varieties under different N application rates and limited water availability can aid in the development of a reasonable fertilization system to promote sustainable agricultural development, further maintaining crop yield and N absorption, and reducing soil erosion hazards. In this study, a two-year field experiment was conducted on winter wheat at three locations to determine the effects of the use of varieties and N application rates on wheat growth, grain yield, grain N accumulation, and net return. Additionally, the appropriate winter wheat varieties and N fertilization ranges to simultaneously optimize the grain yield, grain N accumulation, and net return were determined. The results of this study provide a technical reference for the production of high winter-wheat yields while promoting the synergistic utilization of varieties and nutrients in similar plains around the world.

Materials and methods

Experimental site description

A field experiment was conducted in three representative locations in the Piedmont plain during the 2018–2019 and 2019–2020 winter wheat growing seasons in Gaocheng Liujiazhuang Village (37°96’ N, 114°89’ E), Xinle Zhongtong Village (38°40’ N, 114°71’ E), and Zhaoxian Northern Agricultural Park (38°40’ N, 114°71’ E). The study site in Gaocheng has a temperate, semi-humid, and continental monsoon climate, with an annual average temperature of 12.5°C and an average annual precipitation of 494.0 mm. On the other hand, the study site in Xinle is located at the eastern foot of the Taihang Mountains, on the sloped plain in front of the mountains. The site has a warm temperate, semi-humid monsoon continental climate, with an average temperature of 12.3°C and an average annual precipitation of 428.9 mm. In turn, the study site in Zhaoxian is located on the Piedmont alluvial plain in the middle section of the eastern foothills of the Taihang Mountains. This site has a warm temperate, semi-humid monsoon continental climate with an annual average temperature of 13°C and an average annual precipitation of 502.5 mm. The total amounts of precipitation during the wheat growing season in 2018–2019 and 2019–2020 were 94.4 mm and 135.2 mm at Zhaoxian, 111.4 mm and 112.0 mm at Gaocheng, and 81.3 mm and 139.4 mm at Xinle, respectively. Monthly total precipitations and mean temperatures are shown in Fig 1. The main characteristics of the 0–20 cm and 0–40 cm soil layers in the experimental region at the beginning of the experiment are shown in Table 1.

Fig 1

Daily rainfall (RAIN), maximum temperature (TMAX) and minimum temperature (TMIN) in three experimental sites during 2018–2020 seasons.

Table 1

The status of top soil before seeding in 2018 and 2019.

Growing season	Soil layer(cm)	Experiment Location	Soil bulk density (g cm-3)	Organic matter (g kg-1)	Available N (mg kg-1)	Available P (mg kg-1)	Available K (mg kg-1)
2018–2019	0–20	Gaocheng	1.64	21.30	146.42	22.50	169.60
		Xinle	1.78	19.58	118.42	10.40	108.20
		Zhaoxian	1.66	20.42	135.92	22.10	152.30
	20–40	Gaocheng	1.67	5.33	60.00	5.20	146.80
		Xinle	1.76	4.89	56.58	1.21	91.70
		Zhaoxian	1.73	5.02	53.08	2.70	111.20
2019–2020	0–20	Gaocheng	1.65	20.73	140.75	22.59	162.92
		Xinle	1.72	17.58	118.73	10.91	112.95
		Zhaoxian	1.68	18.98	133.43	21.53	149.99
	20–40	Gaocheng	1.69	5.05	56.70	5.33	147.76
		Xinle	1.75	4.44	53.57	1.42	94.73
		Zhaoxian	1.74	4.87	51.58	2.61	114.26

Experimental design

The experiment adopted a two-factor split plot design at three experimental locations. The main plots were arranged for varieties KN199 (low nitrogen tolerant and high yield) and JM585 (nitrogen sensitive) previously studied [21]. The subplots were arranged for N application with five N fertilizer rates, namely, 0, 120, 180, 240, and 300 kg N ha-1 (referred to as N0, N120, N180, N240, and N300, respectively). Main plots were 50 m long ×10 m wide and subplots were 9 m long ×5 m wide in size, with a 1.0-m buffer zone between plots to minimize the effects of adjacent plots. In sum, there were 30 treatments with three replicates each. Both wheat varieties were sown at a rate of 180 kg seed ha-1. At the third-leaf growth stage, seedlings are thinned to attain a plant population density of 3.3 million basic seedlings per hectare. In the 2018–2019 winter growing season, plots in Gaocheng, Xinle, and Zhaoxian were planted on October 11, 4, and 7, respectively, and the crops were harvested on June 11, 4, and 7, respectively. Meanwhile, in the 2019–2020 growing season, plots in Gaocheng, Xinle, and Zhaoxian were planted on October 10, 10 and 11, respectively, and the crops were harvested on June 15, 10 and 11, respectively. This experiment was carried out under limited water conditions: during the whole growing season, only 60 mm water were applied at the jointing stage. N fertilizer was spread before irrigation at jointing stage, a water meter was connected at the water outlet of the pump, then the water pipe was connected to the plots in order to strictly control water quantity at 60 mm.

Nitrogen, phosphorus and potassium fertilizers were applied using urea (46.7% N), calcium superphosphate (16% P2O5), and potassium sulfate (60% K2O), respectively. The base fertilization before seeding was applied by spreading the fertilizer on the surface and mixing it into the soil with a rotary cultivator. P and K fertilizers were applied at 120 kg ha-1 and 150 kg ha-1, respectively, as base fertilizers; in turn, N fertilizer was split in two portions applied as base fertilizer (50%) and topdressing (50%) at the jointing stage. Herbicides (spray 15% Tribenuron 10 g hm-2) were applied before sowing and pesticides (mix 10% imidacloprid with 50 kg of water and spray) were applied at flowering period, so that the plots were kept free of weeds, insects, and diseases during the growing seasons.

Measurements and calculations

Leaf area index (LAI)

At anthesis, the leaf area of 20 representative plants from each plot was measured using a LI-3000C portable leaf area meter (LI-COR, Lincoln Nebraska, USA).

Aboveground biomass

At maturity, 20 representative plants were selected from each plot. Each plant was divided into grain, stem and sheath, leaves, and the spike axis and glumes, and oven-dried at 105°C for 30 min and subsequently dried at 75°C to constant weight. The plant mass was determined using an electronic balance. The total accumulation of dry matter of each organ per hectare was calculated by sampling the number of plants and the basic seedling number per hectare.

Grain yield and harvest index

At grain maturity, an area of 2 m2 of undisturbed wheat was randomly selected to measure yield. Grains were air-dried and weighed after harvest, samples were taken and oven-dried for measuring moisture content. Yields were normalized at 13% moisture content.

N accumulation, N harvest index and N use efficiency

The various organs of the wheat plants whose dry matter was measured at maturity were ground to determine N accumulation. Dried samples were ground, extracted with H2SO4-H2O2, and analyzed for total N content (%) using a continuous-flow auto-analyzer (AutoAnalyzer 3; Bran + Luebbe, Noderstedt, Germany) [22].

Total N accumulation, N harvest index (NHI) and NUE were calculated according to the method described by Zhang et al. [23].

Net return

Net return was calculated as follows [24]: where, Nr is the net return (CNY ha-1), Gp is the gross profit (CNY ha-1), Ic is the irrigation cost (CNY ha-1), Fc is the fertilizer cost (CNY ha-1), and O represents other costs (CNY ha-1).

Data analysis

Analysis of variance (ANOVA) was performed using IBM SPSS Statistics for Windows, version 25 (IBM Corp., Armonk, NY, USA). Variety and fertilization rate were used as the primary effects and two-way interactions were included as well. All treatment means were compared for significant differences using Duncan’s multiple range test at a significance level of P = 0.05 [25].

Results

Leaf area index, aboveground biomass, grain yield, and harvest index

Location, variety and N rate interaction significantly affected LAI in 2018–2020. The interaction of L×V significantly affected the LAI in 2018–2020, and L×V only affected the LAI in 2019–2020 (Table 2). As shown in Fig 2, LAI recorded for KN199 under the N180 treatment increased significantly by 24.11%, 63.10%, and 45.79% at Gaocheng, Xinle, and Zhaoxian, respectively, compared with the N0’treatment in 2018–2019 growing season. In 2018–2019, LAI for JM585 under the N180 treatment increased significantly by 15.08%, 82.89%, and 42.11% at Gaocheng, Xinle, and Zhaoxian, respectively, compared with the N0’treatment. Moreover, in 2019–2020, at Xinle and Zhaoxian experimental locations, the LAI of JM585 increased significantly by 4.34% and 11.95%, respectively when the N input increased from N180 to N240.

Table 2

Significance level (P values) of the effects of location, variety and N rate on leaf area index, aboveground biomass, grain yield, HI, total N accumulation, grain N accumulation, NHI and NUE of winter wheat.

Growing season	Treatment	Leaf area index	Aboveground biomass	Grain yield	HI	Total N accumulation	Grain N accumulation	NHI	NUE
2018–2019	Location (L)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	Variety (V)	<0.001	<0.001	0.021	<0.001	0.547	<0.001	<0.001	0.017
	N rate (N)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	L×V	<0.001	<0.001	0.536	<0.001	<0.001	<0.001	<0.001	0.005
	L×N	0.983	<0.001	0.090	<0.001	<0.001	<0.001	<0.001	<0.001
	V×N	0.365	0.001	0.845	0.196	<0.001	<0.001	0.099	<0.001
	L×V×N	0.433	0.003	0.999	0.021	0.128	0.178	0.013	<0.001
2019–2020	Location (L)	<0.001	<0.001	<0.001	0.170	<0.001	<0.001	<0.001	<0.001
	Variety (V)	<0.001	0.004	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	N rate (N)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	L×V	<0.001	0.005	<0.001	<0.001	0.557	0.005	<0.001	<0.001
	L×N	<0.001	0.002	0.042	0.093	<0.001	<0.001	<0.001	<0.001
	V×N	0.820	0.017	0.426	0.170	0.132	0.302	0.832	0.018
	L×V×N	0.199	0.003	0.101	0.849	0.003	0.003	0.021	0.471
2018–2020	Growing season (S)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	Location (L)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	Variety (V)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	N rate (N)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	S×L	<0.001	<0.001	<0.001	0.001	<0.001	<0.001	<0.001	<0.001
	S×V	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
	S×N	<0.001	<0.001	<0.001	0.005	<0.001	0.216	<0.001	0.019
	L×V	<0.001	<0.001	<0.001	<0.001	0.001	<0.001	<0.001	<0.001
	L×N	<0.001	<0.001	0.002	0.004	<0.001	<0.001	<0.001	<0.001
	V×N	0.378	<0.001	0.313	0.023	<0.001	<0.001	0.911	0.036
	S×L×V	<0.001	0.143	<0.001	<0.001	0.109	0.264	0.013	<0.001
	S×L×N	<0.001	<0.001	0.672	0.007	<0.001	<0.001	<0.001	0.197
	S×V×N	0.686	0.856	0.961	0.871	0.033	0.078	0.110	<0.001
	L×V×N	0.466	0.053	0.399	0.352	0.005	0.009	0.017	<0.001
	S×L×V×N	0.265	<0.001	0.522	0.188	0.020	0.013	0.013	<0.001

Note: HI: Harvest index; NHI: Nitrogen Harvest index; NUE: Nitrogen use efficiency.

Fig 2

Leaf area index at anthesis of different winter wheat varieties under different nitrogen fertilization rates in three experimental sites during 2018–2020 seasons.

Error bars represent one standard deviation from the mean. Letters above the bars are comparison results of leaf area index between different nitrogen treatments of the same variety. There is no significant difference between treatments with same letters.

Location, variety, and N rate significantly affected aboveground biomass in the two growing seasons considered herein. The interaction of L×V, L×N and V×N significantly affected the aboveground biomass in 2018–2020 (Table 2). As shown in Fig 3, the aboveground biomass of KN199 under the N180 treatment increased significantly by 23.12%, 25.60%, and 17.66% at Gaocheng, Xinle, and Zhaoxian, respectively, compared with the N0’treatment in 2018–2019 growing season. In the 2018–2019 growing season, the aboveground biomass of JM585 increased significantly by 24.69%, 26.56%, and 20.37% at Gaocheng, Xinle, and Zhaoxian, respectively, under the N180 treatment, compared to the N0’treatment. Furthermore, when N input increased to N240 and N300, aboveground biomass significantly increased in Xinle and Zhaoxian experimental locations; However, grain yield did not increase significantly, while dry matter accumulated in vegetative organs was the portion of the plant body whose dry matter increased more. In the 2019–2020 growing season, grain yield and dry matter for the two winter wheat varieties were consistent with those observed in the previous season.

Fig 3

Aboveground biomass at maturity of different winter wheat varieties under different nitrogen fertilization rates in three experimental sites during 2018–2020 seasons.

Error bars represent one standard deviation from the mean. Letters above the bars are comparison results of leaf area index between different nitrogen treatments of the same variety. There is no significant difference between treatments with same letters.

Location, variety, and N rate significantly affected grain yield in the two growing seasons considered herein. The interaction of L×V significantly affected the grain yield in 2019–2020 (Table 2). Thus, as shown in Fig 4, grain yield of KN199 increased significantly by 14.70%, 14.07%, and 9.16% at Gaocheng, Xinle, and Zhaoxian, respectively, under N180 compared to the N0’treatment in 2018–2019 growing season. Similarly, in 2018–2019, grain yield of JM585 increased significantly by 16.99%, 18.96%, and 9.72% at Gaocheng, Xinle, and Zhaoxian, respectively, under N180 compared to the N0’treatment. Furthermore, in 2019–2020, the grain yield variation observed for the two winter wheat varieties evaluated was consistent with that observed in the 2018–2019 season. On the other hand, in 2019–2020 at Gaocheng, the grain yield of KN199 and JM585 under the N0–N300 treatments significantly lower than 2018–2019. No significant differences in grain yield were detected among N180, N240, and N300 in either growing season.

Fig 4

Grain yield of different winter wheat varieties under different nitrogen fertilization rates in three experimental sites during 2018–2020 seasons.

Error bars represent one standard deviation from the mean. Letters above the bars are comparison results of leaf area index between different nitrogen treatments of the same variety. There is no significant difference between treatments with same letters.

Variety and N rate significantly affected HI in 2018–2020 while location significantly affected HI in 2018–2019. The interaction of L×V significantly affected the HI in 2018–2020, and L×N only affected the HI in 2019–2020 (Table 2). As shown in Fig 5, HI was highest under N0 and decreased as the N application rate increased. The range of HI values observed in 2018–2019 for KN199 under N0–N300 at Gaocheng, Xinle, and Zhaoxian, was 0.48–0.50, 0.47–0.50, and 0.47–0.51, respectively. Similarly, in 2018–2019, the range HI values observed that same year for JM585 HI was 0.45–0.46, 0.45–0.47, and 0.45–0.50, respectively. In 2019–2020, HI values for the two varieties of winter wheat were consistent with those observed in 2018–2019. HI values for KN199 were higher than those for JM585 across N application rates.

Fig 5

Harvest index of different winter wheat varieties under different nitrogen fertilization rates in three experimental sites during 2018–2020 seasons.

Error bars represent one standard deviation from the mean. Letters above the bars are comparison results of leaf area index between different nitrogen treatments of the same variety. There is no significant difference between treatments with same letters.

Winter wheat nitrogen accumulation, N harvest index, and NUE

Location and N rate significantly affected total N accumulation in 2018–2020 while variety significantly affected total N accumulation in 2019–2020. The interaction of L×V significantly affected the total N accumulation in 2018–2020, and L×N, V×N only affected the total N accumulation in 2018–2019. Overall, location, variety, and N rate significantly affected grain N accumulation in 2018–2020. The interaction of L×V and L×N significantly affected the grain N accumulation in 2018–2020, and V×N only affected the total N accumulation in 2018–2019. Location, variety, and N rate significantly affected NHI in 2019–2020. The interaction of L×V and L×N significantly affected the NHI in 2018–2020. Location, variety and N rate in 2018–2020. The interaction of L×V, L×N and V×N significantly affected NUE in 2018–2020 (Table 2).

As shown in Table 3, total N accumulation in KN199 increased significantly by 28.09%, 28.00%, and 37.35% at Gaocheng, Xinle, and Zhaoxian, respectively, under N180 compared to the N0’treatment in 2018–2019 growing season. Similarly, in 2018–2019, total N accumulation in JM585 increased significantly by 40.16%, 34.83%, and 45.68% at Gaocheng, Xinle, and Zhaoxian, respectively, under N180 compared to the N0’treatment. Furthermore, when N input increased to N240, total N accumulation significantly increased in the three experimental locations. When N input increased to N300, the total N accumulation did not increase significantly. Grain N accumulation in KN199 increased significantly by 23.97%, 23.76%, and 28.41% at Gaocheng, Xinle, and Zhaoxian, respectively, under N180 compared to the N0’treatment in 2018–2019 growing season. Similarly, in 2018–2019, grain N accumulation in JM585 increased significantly by 40.16%, 34.83%, and 45.68% at Gaocheng, Xinle, and Zhaoxian, respectively, under N180 compared to the N0’treatment. NHI for KN199 was 0.86–0.83, 0.89–0.85, and 0.89–0.81 at Gaocheng, Xinle, and Zhaoxian, respectively, while NHI for JM585 was 0.85–0.81, 0.86–0.83, and 0.86–0.79 at Gaocheng, Xinle, and Zhaoxian, respectively. Furthermore, in 2019–2020, total N accumulation, grain N accumulation, and NHI and NUE variation for the two winter wheat varieties tested were consistent with the corresponding values obtained in the previous season. Total N accumulation and grain N accumulation increased with increasing fertilization rates, whereas NHI and NUE decreased with increasing fertilization rates.

Table 3

Nitrogen accumulation at maturity of different winter wheat varieties under different nitrogen fertilization rates in three experimental sites during 2018–2020 seasons.

2018–2019							2019–2020
Treatment			Total nitrogen accumulation (kg ha-1)	Grain nitrogen accumulation (kg ha-1)	N harvest index	NUE	Total nitrogen accumulation (kg ha-1)	Grain nitrogen accumulation (kg ha-1)	N harvest index	NUE
Location	Variety	N rate
Gaocheng	KN199	N0	208.17d	178.19d	0.86a	34.36a	201.26d	171.25c	0.85a	33.18a
		N120	231.40c	195.04c	0.84b	33.00b	242.03c	190.82b	0.79b	29.81b
		N180	266.65b	220.90b	0.83c	32.91b	258.91b	198.13b	0.77c	28.91bc
		N240	292.54a	243.09a	0.83c	30.44c	283.22a	213.29a	0.75d	28.14c
		N300	296.58a	244.71a	0.83c	30.40c	291.60a	217.45a	0.75d	28.11c
	JM585	N0	201.29d	170.64c	0.85a	34.41a	181.94d	148.78d	0.82a	32.66a
		N120	247.49c	204.98b	0.83b	31.95b	222.93c	174.15c	0.78b	30.35b
		N180	282.13b	232.52a	0.82b	30.24c	258.54b	192.22b	0.74c	28.26c
		N240	295.50a	240.03a	0.81c	29.57c	281.94a	207.79a	0.74c	27.69c
		N300	300.91a	243.06a	0.81c	29.20c	287.73a	210.27a	0.73c	27.48c
Xinle	KN199	N0	143.98d	128.34d	0.89a	38.07a	148.78d	124.87d	0.84a	35.87a
		N120	169.38c	148.08c	0.87b	35.04b	186.70c	154.67c	0.83b	32.17b
		N180	184.30b	158.84b	0.86c	35.05b	208.74b	167.43b	0.80c	32.61b
		N240	203.01a	173.65a	0.86c	32.81c	245.31a	195.19a	0.80c	28.84c
		N300	208.88a	178.14a	0.85c	32.35c	249.41a	196.26a	0.79d	28.46c
	JM585	N0	134.23d	116.06d	0.86a	38.26a	138.10d	113.82d	0.82a	34.85a
		N120	169.12c	145.17c	0.86a	35.54b	183.43c	146.65c	0.80b	32.37b
		N180	180.98b	154.68b	0.85b	35.40b	209.17b	163.54b	0.78c	30.56c
		N240	197.95a	167.60a	0.85b	33.02c	223.92a	170.77a	0.76d	28.43d
		N300	201.67a	170.05a	0.84c	32.03c	226.02a	171.64a	0.76d	28.52d
Zhaoxian	KN199	N0	164.44d	145.98c	0.89a	39.89a	167.47d	142.30c	0.85a	41.99a
		N120	184.18c	160.56b	0.87b	38.87b	205.01c	172.88b	0.84a	35.27b
		N180	225.87b	187.46a	0.83c	32.38c	259.01b	208.55a	0.81b	32.39c
		N240	241.76ab	196.98a	0.81d	31.35d	276.01a	215.54a	0.78c	30.98c
		N300	243.60a	198.11a	0.81d	30.90d	279.44a	216.17a	0.77c	30.75c
	JM585	N0	152.77d	131.23c	0.86a	42.29a	155.75d	126.84c	0.81a	37.78a
		N120	200.17c	167.37b	0.84b	34.26b	196.83c	154.54b	0.78b	32.47b
		N180	222.56b	178.18a	0.80c	32.21c	246.35b	184.17a	0.75c	29.01c
		N240	228.46ab	181.36a	0.79d	32.13c	258.87ab	188.80a	0.73d	28.02cd
		N300	237.42a	187.44a	0.79d	31.24c	269.72a	194.66a	0.72d	27.22d

Note: Different letters following data of same trait and variety indicate significant differences between nitrogen treatments (P < 0.05).

Economic return

As shown in Table 4, In 2018–2020, water cost and cost of cultivation for two winter wheat varieties under five N treatment being same at three experiment location. Fertilizer cost increased as fertilizer rate increased in both winter wheat varieties. In 2018–2020, fertilizer cost for two winter wheat varieties under the N180 treatment being higher by 13%, 50% than N0, N120, being lower by 10%, 22% than N240, N300 treatment at three experiment location. Gross profit obtained from the two tested winter wheat varieties increased with N application rate across locations and over the two growing seasons. However, no significant further increases in gross profit were observed when N input exceed to 180 kg ha-1. Net return first increased and then decreased as fertilizer rate increased in both winter wheat varieties. In 2018–2019, net return obtained from KN199 was significantly the highest under the N180 treatment being higher by 1.49%–13.76%, 1.20%–16.21% than other treatment at Gaocheng and Xinle, under the N120 treatment being higher by 0.53%–6.14% than other treatment at Zhaoxian. Net return obtained from JM585 was significantly the highest under the N180 treatment being higher by 3.41%–19.25%, 3.36%–27.14%, 0.56%–6.85% than other treatment at Gaocheng, Xinle and Zhaoxian. As for 2019–2020, net return obtained from KN199 was significantly the highest under the N180 treatment being higher by 3.49%–20.48%, 3.79%–77.32%, 1.84%–23.13% than other treatment at Gaocheng, Xinle and Zhaoxian. Net return obtained from JM585 was significantly the highest under the N180 treatment being higher by 4.33%–66.96%, 3.02%–68.38%, 1.25%–26.41% than other treatment at Gaocheng, Xinle and Zhaoxian.

Table 4

Economic return (CNY ha-1) of different winter wheat varieties under different nitrogen fertilization rates in three experimental sites during 2018–2020 seasons.

2018–2019								2019–2020
Treatment			Water cost	Fertilizer cost	Cost of cultivation	Gross profit	Net return	Water cost	Fertilizer cost	Cost of cultivation	Gross profit	Net return
Location	Variety	N rate
Gaocheng	KN199	N0	1060a	1725e	3600a	16778b	10393b	1060a	1725e	3600a	12889b	6504b
		N120	1060a	2297d	3600a	18519a	11562a	1060a	2297d	3600a	14333a	7376ab
		N180	1060a	2585c	3600a	19244a	11999a	1060a	2585c	3600a	15081a	7836a
		N240	1060a	2873b	3600a	19356a	11823a	1060a	2873b	3600a	15105a	7572ab
		N300	1060a	3159a	3600a	19174a	11355a	1060a	3159a	3600a	15096a	7277ab
	JM585	N0	1060a	1725e	3600a	16348c	9963c	1060a	1725e	3600a	11185c	4800c
		N120	1060a	2297d	3600a	17889b	10932b	1060a	2297d	3600a	13230b	6273bc
		N180	1060a	2585c	3600a	19126a	11881a	1060a	2585c	3600a	15259a	8014a
		N240	1060a	2873b	3600a	19022a	11489ab	1060a	2873b	3600a	15214a	7681ab
		N300	1060a	3159a	3600a	19154a	11335ab	1060a	3159a	3600a	15207a	7388ab
Xinle	KN199	N0	1060a	1725e	3600a	11319b	4934a	1060a	1725e	3600a	10257c	3872b
		N120	1060a	2297d	3600a	12533a	5576a	1060a	2297d	3600a	13029b	6072a
		N180	1060a	2585c	3600a	12911a	5666a	1060a	2585c	3600a	14111a	6866a
		N240	1060a	2873b	3600a	13267a	5734a	1060a	2873b	3600a	14148a	6615a
		N300	1060a	3159a	3600a	13148a	5329a	1060a	3159a	3600a	14096a	6277a
	JM585	N0	1060a	1725e	3600a	10667b	4282a	1060a	1725e	3600a	9968c	3583c
		N120	1060a	2297d	3600a	12067a	5110a	1060a	2297d	3600a	12111b	5154b
		N180	1060a	2585c	3600a	12689a	5444a	1060a	2585c	3600a	13278a	6033a
		N240	1060a	2873b	3600a	12800a	5267a	1060a	2873b	3600a	13389a	5856ab
		N300	1060a	3159a	3600a	12884a	5065a	1060a	3159a	3600a	13252a	5433ab
Zhaoxian	KN199	N0	1060a	1725e	3600a	14067b	7682a	1060a	1725e	3600a	13074b	6689b
		N120	1060a	2297d	3600a	15111a	8154a	1060a	2297d	3600a	15044a	8087a
		N180	1060a	2585c	3600a	15356a	8111a	1060a	2585c	3600a	15481a	8236a
		N240	1060a	2873b	3600a	15452a	7919a	1060a	2873b	3600a	15513a	7980a
		N300	1060a	3159a	3600a	15450a	7631a	1060a	3159a	3600a	15496a	7677a
	JM585	N0	1060a	1725e	3600a	13941b	7556a	1060a	1725e	3600a	10630c	4245b
		N120	1060a	2297d	3600a	14963a	8006a	1060a	2297d	3600a	11963b	5006ab
		N180	1060a	2585c	3600a	15296a	8051a	1060a	2585c	3600a	12611ab	5366a
		N240	1060a	2873b	3600a	15430a	7897a	1060a	2873b	3600a	12833a	5300a
		N300	1060a	3159a	3600a	15354a	7535a	1060a	3159a	3600a	12666ab	4847ab

Note: Different letters following data of same trait and variety indicate significant differences between nitrogen treatments (P < 0.05).

Optimization of variety and N application rate based on yield and economic return

The data collected in this study over two growing seasons were comprehensively analyzed by considering the effects of different fertilization rates. N application rate was treated as an independent variable, and grain yield and net return were considered as response variables. The data were analyzed using the fertilizer effect function, and a unitary quadratic regression equation was determined, which was used to calculate the fertilizer amount required to maximize grain yield and net return (Fig 6). In 2018–2019, the maximum yield from KN199 was achieved with N application rates of 247.0, 285.5, and 254.2 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively. In turn, net return was maximized with the application of 190.5, 192.2, and 144.7 N kg ha-1 at Gaocheng, Xinle and Zhaoxian, respectively. In the same period, the maximum yield from JM585 was achieved with N application rates of 290.1, 284.8, and 257.5 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively, while net return in this case was maximized with the application of 220.1, 246.9, and 150.3 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively. In turn, in the 2019–2020 growing season, the maximum yield from KN199 was achieved with N application rates of 264.6, 250.6, and 238.6 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively, while net return was maximized with the application of 191.2, 212.9, and 184.5 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively. Meanwhile, in the same period, the maximum yield from JM585 was achieved with N application rates of 295.8, 262.5, and 266.7 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively, while net return was maximized with the application of 223.3, 214.8, and 188.8 N kg ha-1 at Gaocheng, Xinle, and Zhaoxian, respectively.

Fig 6

Correlation of winter wheat grain yield and economic return to nitrogen fertilization rates.

When the N application rate exceeded 180 kg ha-1, grain yield did not increase significantly. Furthermore, at rates lower than the optimal N application rate, the net return of winter wheat increased with the application rate; however, when higher than the optimal rate, the return gradually decreased. Altogether, data indicated that the optimal N application rate in the Piedmont plain of the Taihang Mountains in China, ranged from 144.7 to212.9 kg ha-1 for KN199 and from 150.3 to 247.0 kg ha-1 for JM585.

Discussion

Effects of nitrogen application rate on LAI, aboveground biomass, grain yield, and harvest index of winter wheat

Variety, amount of irrigation, and fertilizer available for uptake directly affect the growth and development of wheat, thereby affecting final grain yield [26,27]. Suitable irrigation amount, N application rate, and variety can significantly increase LAI, aboveground biomass, and grain yield of winter wheat, while N uptake increases with increasing irrigation and N fertilization [28,29]. Previous studies found that LAI, biomass production, grain yield, and yield are positively affected by N fertilization, but negatively affected by water stress [30,31]. In the experiments reported herein, N application rate affected grain yield and LAI of the genotypes under evaluation; furthermore, genotypic responses varied with environment, i.e., with site and year [32]. LAI and aboveground biomass were significantly higher under the N180 treatment than under the N0’treatment across locations. When N input increased to N240, there were no significant differences in KN199, and when increased to N300, there were no significant differences in JM585 with respect to LAI or aboveground biomass. When the N application rate exceeded 180 kg ha-1, grain yield did not increase significantly; as what increased mostly was the dry matter accumulated in vegetative organs, while HI decreased concomitantly. Guttieri et al. [33] studied the effect of N fertilization on the grain yield of different genotypes. Yue et al. [34] found that wheat grain yield increased with an increase in N supply but excess N did not increase grain yield nor grain N accumulation in five experimental sites. Consistently, Zhang et al. [35] showed that 190 kg N ha-1 can be applied in Beijing, China, to maintain a steady yield for at least two years. In this study, grain yield increased with increasing N application rate but when it exceeded 180 kg ha-1, the yield of the two varieties tested did not increase significantly. In 2019–2020, at the Gaocheng experimental location, the grain yield for KN199 and JM585 decreased by 21.27–23.18% and 20.02–26.69%, respectively, compared with the grain yield recorded in the 2018–2019 growing season. This was attributed to a reduction in the number of grains per panicle owing to freezing damage at the panicle differentiation stage and continuous rain during the flowering period (Fig 1).

Effects of application rate on N accumulation and NUE

Excess N application results in reduced NUE and soil pollution [36]. Improving NUE facilitates the rational utilization of agricultural resources [37]. Zheng et al. [18] concluded that N accumulation in winter wheat increases with increasing N application rate; however, grain N accumulation decreases at N application rates above 240 kg N ha-1. In this study, total N accumulation and grain N accumulation increased with increasing N application rate and when the latter exceeded 240 kg ha-1, they remained increased in a non-significant way. Presumably because of differences in planting area and wheat variety. In addition, the results of this study showed that the N240 and N300 treatments increased total plant N uptake; however, grain N uptake and grain yield did not increase. Indeed, our results showed that maximum NHI was obtained under the N0’treatment in 2019–2020 at the three experimental sites; yet, conversely, the accompanying yields were the lowest. This may be due to frost damage during the panicle differentiation period and continuous rain during the flowering period of 2019–2020 (Fig 1), the number of grains per spike was reduced, which resulted in a decrease in grain yield. Several studies have reported a NUE below 30% for winter wheat [18,38]. Studies have shown that wheat varieties differ for NUE; high-N varieties (such as varieties with high uptake efficiency and utilization efficiency) also have high plant dry matter yields and grain yields under conditions of insufficient N supply [39]. In this study, NUE generally decreased and differed only slightly with increasing N application rate across sites in both varieties; furthermore, it decreased in both with an increasing N application rate. NUE of KN199 and JM585 ranged within 30.4–39.89 and 28.11–41.99, and 29.2–42.29 and 27.22–37.78 in the 2018–2019 and 2019–2020 growing seasons, respectively. As it can be seen, NUE was higher in the first than in the second growing season, mainly owing to the high rainfall that caused the accumulation of dry matter in plant vegetative organs in the latter. Our results demonstrated that excessive N fertilization reduced grain N uptake, grain yield, and NUE. Therefore, environment-dependent site-specific genotype and N application rate recommendations should be promoted, in accordance to prevailing environmental conditions or seasonal expectations.

Combined effects of variety and nitrogen application rate

Many researchers have elucidated the relationships among amount of irrigation, fertilizer input, and crop yield using a combination of multivariate regression and spatial analyses [40,41]. The optimal N application rate for wheat varies with soil conditions in the planting area. Previous studies on wheat have shown that an irrigation of 240 mm and an N application rate of 150–210 kg N ha-1 can maintain higher productivity in the entire system with higher resource use efficiency and lower N2O emissions in the NCP [11]. In this study, the relationships between fertilizer cost and crop yield were further analyzed and the net return for the maximum values of these parameters was assessed at three experimental sites. The study data suggest that the optimal N fertilizer rates to use at the Piedmont plain of the Taihang Mountains for the sustainable production of winter wheat varieties KN199 and JM585 are 144.7–212.9 and 150.3–247.0 kg ha-1, respectively.

Conclusions

Under limited irrigation conditions, LAI, dry matter accumulation, grain yield, and nitrogen accumulation of the two evaluated wheat varieties in the study sites increased with increasing N application rate; however, HI, NHI, and NUE decreased. The optimal fertilizer amounts for winter wheat varieties KN199 and JM585 varied within the ranges of 144.7–212.9 and 150.3–247.0 kg ha-1, respectively, as these conferred the highest economic return, while facilitating water and fertilizer conservation, decreasing groundwater pollution risk, and maintaining high grain yield. The results of this study are of great significance for the scientific management of winter wheat fertilization in the Piedmont plains of the Taihang Mountains of China, which may be similar to many other arid and semi-arid winter wheat production areas around the world.

(XLSX)

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7 Sep 2021

PONE-D-21-25804Optimizing nitrogen fertilizer amount for best performance and highest economic return of winter wheat under limited water irrigation conditionsPLOS ONE

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Reviewer #1: Recommendation: Major revision

The survey reports an interesting topic that points out the necessity of nitrogen fertilization optimization in the fields of winter wheat under limited irrigation and aiming a higher economic return. The manuscript is written in a good standard English style and a good statistical approach has been presented. I think the manuscript is of good quality and has a merit to be published in PLOS One when the recommended revision is successfully done. Following, a list of comments that will help authors improve their manuscript.

1. Page 2, line 28: Kindly remove “a” before few.

2. Page 3, line 40: Kindly add a space between “population” and “[1]”.

3. Page 3, lines 42–43: Kindly provide a reliable source (reference) for this statement.

4. Page 3, line 44: Kindly remove “mush too frequently” and adjust as follow: “farmers frequently use…”.

5. Page 4, line 58: Kindly adjust as follow: “is reported”.

6. Page 4, line 64: Kindly adjust as follow: “[16,17].

7. Page 5, line 67: Kindly define the abbreviation “NUE” at its first mention so that the reader can understand that you’re talking about the nitrogen use efficiency.

8. Page 5, line 68: Kindly add a space between “costs” and “[19]”.

9. Page 5, lines 74–78: I support the idea, but try to reformulate as the sentence is too long.

10. Page 6, lines 83–88: Kindly use the impersonal form for these sentences and avoid the first voice form.

11. Page 8, line 120: I’m okay with the arrangement; however, we can’t rely on unpublished works as a key reference.

12. Page 8, line 124: Kindly replace “In all” by “In sum”.

13. Page 9, lines 128, 130: Kindly adopt an homogeny term: either “growing season” or “cropping season”.

14. Page 9, line 131: Kindly add “respectively” after “11”.

15. Page 9, line 134: Kindly even use the atomic form of nitrogen, phosphorus, and potassium or their full name.

16. Page 9, line 143: Kindly put “Leaf area index (LAI)” as a subtitle in italic form.

17. Page 9, line 143: Kindly replace “in” by “from”.

18. Page 10, line 146: Same recommendation as point 13.

19. Page 10, line 152: Same recommendation as points 13 and 18.

20. Page 11, line 169–170: Kindly adjust as follow: “fertilizer cost”, and “other costs”.

21. Page 12, line 179: I suggest writing the full name of LAI here (leaf area index).

22. Page 12, line 180: You mean the single effect of location, variety and N rate not their combined effect (interaction); as the interaction of these three factors has no significant effect on the leaf area index in both growing seasons 2018–2019 and 2019–2020.

23. Page 12, line 183: Kindly add “in both growing seasons” after “N0 treatment”.

24. Page 12, lines 186–188: “However… treatments”: Kindly remove this section as discussing non significant results is not important.

25. Page 12, line 188: Kindly replace “However” by “Moreover”.

26. Page 12, line 189: Kindly adjust as follow: “in Xinle and Zhaoxian experiment locations”. At what growing season ?

27. Page 12, line 190: In comparison with which N treatment ? N0 ?

28. Page 12, line 191: Kindly remove “although… N300” for the same reason mentioned in point 24.

29. Page 13, Table 2: You may point out the interaction between location and variety in both seasons on leaf area index by naming it (L ˟ V) and the interaction between location and N rate in 2019–2020 season (L ˟ N).

30. Page 13, Table 2: Kindly define each abbreviation as a table footnote.

31. Page 13, line 195: Kindly remove “Location, variety, and N rate in 2018–2020 (Table 2).”

32. Page 13, line 195, Figure 3: Kindly correct the order of the locations graphs to be similar in the upper and the lower rows and capitalize the first letter of all locations names.

33. Page 13, line 196: In which growing season ?

34. Page 13, line 198: Kindly adjust as follow: “growing season”.

35. Page 13, line 201: Kindly adjust as follow: “N0’s one”.

36. Page 13, line 202: Kindly add that it was a significant increase in Xinle and Zhaoxian experimental locations.

37. Page 13, line 203: Same recommendation as in points 13, 18, and 19.

38. Page 14, line 204: Kindly mention if it was a significant increase in comparison with the control (N0 treatment), in which season, and by how much.

39. Page 14, line 205: Same recommendation as in point 13, 18, 19, and 37.

40. Page 14, line 206: Kindly adjust as follow: “in the previous season”.

41. Page 14, lines 208–210: Kindly mention the growing season.

42. Page 14, lines 213–214: Actually, you can mention in the previous sentences both seasons instead of splitting the idea into two.

43. Page 14, line 215: Kindly mention the effect of the interactions between location and variety, and location and N rate on the grain yield.

44. Page 14, lines 215–216: Actually, it was the case of all treatments in 2019–2020 season. It looks like it is correlated with environmental changes rather that fertilization effect.

45. Page 14, lines 217–218: Same recommendation as in points 13, 18, 19, 37, and 39.

46. Page 15, lines 219–227: Kindly mention the effect of the interactions between location and variety, and location and N rate in the growing season 2018–2019 on the harvest index.

47. Page 15, line 222: Kindly specify the N treatments.

48. Page 15, lines 223–225: Kindly mention the season and specify the N treatments.

49. Page 15, lines 225–226: Same recommendation as in point 42.

50. Page 15, line 229: Kindly mention the complete name of NUE in the title.

51. Page 15, lines 230–234: There are plenty of non-discussed results especially concerning the interactions between factors location, variety and N rate and their effect on the studied parameters.

52. Page 15, line 233: It is the case in both growing seasons.

53. Pages 15–16, lines 235–245: It is a reading of the results. Kindly try to compare the results rather than reading them.

54. Page 16, lines 246–248: Same recommendation as in points 42 and 49.

55. Pages 16–17, lines 251–252: Good discussion point, but kindly move it to the “Discussion” part.

56. Page 18, line 258: Kindly adjust as follow: “from the two tested winter wheat varieties”.

57. Page 18, line 261: Do you mean by “net return” the “economic return” ? Kindly adopt an homogeny term and mention when it increased and decreased.

58. Page 18, lines 262–270: Kindly follow the recommendation in the next point for this section.

59. Page 19, line 272, Table 4: Kindly make the Duncan test for this table to detect which treatments in both varieties and growing seasons give the most significant gross profit and economic return and lower costs. In the current way provided, we cannot rely on the results to judge.

60. Page 21, lines 299–302: The sentence is too heavy; kindly reformulate.

61. Page 21, lines 302–303: Kindly adjust as follow: “Altogether, data indicated that…”.

62. Page 22, line 311: Kindly adjust as follow: “[25,26]”.

63. Page 22, line 314: Kindly adjust as follow: “[27,28]”.

64. Page 22, line 316: Kindly adjust as follow: “[29,30]”.

65. Page 23, line 328: Add a space between “[34]” and “showed”.

66. Page 23, lines 334–336: Good discussion of the results !!

67. Page 24, lines 339–340: “Improving… resources”: Kindly provide a reliable source (reference) for this statement.

68. Page 24, line 352: Kindly adjust as follow: “[17,36]”.

69. Page 24, line 352: Kindly adjust as follow: “… NUE; high-N varieties…”

70. Page 24, line 353: Kindly mention the full name at the first mention of a parameter, and follow it by the abbreviation between brackets.

71. Page 25, line 359: Kindly adjust as follow: “As it can be seen…”

72. Page 25, lines 359–360: Same recommendation as in points 13, 18, 19, 37, 39, and 45.

73. Page 25, line 361: Kindly adjust as follow: “excessive” or “excess in”.

74. Page 25, line 362: Kindly remove “and” before “site-specific”.

75. Page 25, line 366: Kindly remove “The” from the paragraph title.

76. Page 25, line 369: Kindly adjust as follow: “[38,39]”.

77. Page 26, line 371: Is it an irrigation or a rainfall ? I guess you mean an annual or a seasonal rainfall. Kindly clarify.

78. Page 26, lines 373–378: Kindly use the impersonal form for these sentences.

79. Page 26, lines 381–385: The statement is right but is too long. Kindly reformulate.

80. Pages 26–27, lines 385–389: The statement is right but is too long. Kindly reformulate.

81. Page 27, lines 390–391: Kindly adjust as follow: “… management of winter wheat fertilization in the Piedmont plains…”

Reviewer #2: The topic is not novel; however, the research data covers the research objectives and the trails at various locations make it valuable.

My only concern is why the author didn't use the control (well-watered conditions) to compare the results under limited water conditions?

I will suggest to compare your results with the control (well-watered conditions) as well.

Reviewer #3: This study is not unique in itself as similar studies have been conducted in wheat as can been seen from many of the studies that authors have cited. However, this study does provide valuable and useful scientific information that can be helpful in moving toward sustainable wheat framing in the Piedmont plain region, hence I recommend the editor to consider accepting this manuscript for publication with revisions.

I do find the manuscript is well written and is technically sound but there are certain information that I found to be missing or not explained well enough for me to fully understand all the aspects of the material and method section, which I have listed below. Additionally, I was not been able to see any attached file with primary data (e.g gain yield data) used for various analysis which is required as per PLOS Data policy, please request authors to make those data available as supplemental file or deposited to a public repository, if not done so already.

Revision suggestion:

Material and Methods:

Experimental site description:

Line 115: Table 1 is showing NPK measurement prior to 2018 seeding, was similar measurement obtain for 2019 year before seeding? If yes, please add them

Experimental design

Line 118-124: Please reword and clarify the experimental design description. Was it complete randomized block design? Was the treatment randomized? How many subplots per main plot were there? If the main plot was 50 m. wide and sub-plots were 10m. wide with 1.0 m buffer between subplots then does that mean there were only 4 sub-plots per main plain (ie. Only 4 N treatments per main plot instead of 5 N treatment)

Line 122: What was the reason behind selecting those specific five N fertilizer rates?

Line 125: Was commercially available seed used both year for seeding?

Line 133: Please provide method of irrigation

Line 139: Please mention if topdressing of N was done after or before irrigation at the jointing stage.

Line 140: Provide information on which pesticides and herbicides were applied, mode of application and their dosage.

Measurements and calculations:

Line 153: Was grain moisture estimated after drying and was grain moisture used for adjusting grain yield?

Data analysis:

Line 173-174: Please provide the ANOVA model. Was year included in the model along with the location? If not, then does ANOVA result differ if year is added to the model.

Results:

Line 187-188: Was no significant differences in LAI for 2019-2020 observed due to higher residual N-P-K content in soil due to application? Was same exact field used in both years of the study?

Discussion:

Line 349-351: any specific environmental reason why grain per spike was reduced in 2019-2020?

Line 374-375: Will using available N in soil prior to seeding or fertilizer application improve calculation of required N fertilizer rate for each season more accurately and help improve economic return?

Reviewer #4: This study tried to find the appropriated nitrogen fertilizer on winter wheat with limited water resources. Authors measured the plant parameters and calculated the economic return. After reading this manuscript, I have several concerns. First, I am not sure the novelty of this manuscript. Second, authors should check the reference format/style carefully. Third, I felt that the cited reference in the main text is not the corrected one. Authors should check for it. In addition, authors used two wheat verities which KN199 has the high NUE and JM585 has the low NUE trait. However, in the table 3, there is no different between KN199 and JM585 on NUE. In addition, authors did not explain the table 2 very well which I think this table has lots of meaning. Besides, I have several comments as follows.

1. Authors should check the usage and grammar. For examples, line 44-45.

2. Line 46-49 and 64 and 159, I am not sure that cited references are relevant with the sentence.

3. Line 50, reference 78?

4. Line 70-71 and 74-78, what is the reference?

5. Line 79, what are the N absorption-related traits?

6. Line 88-90, I am not sure that this manuscript would provide the theoretical basis.

7. Line 119-121, authors used two different NUE wheat varieties. However, there is no reference on it. As I mentioned earlier, there is no different between KN199 and JM585 on NUE. Authors should explain for that.

8. Line 167, it should be E instead of Eb

9. In the result section, authors used the combined data in the text but used the separate data in the table. It is hard to judge the number.

10. Line 233, it should be 2018-2020.

11. Line 238, it should be 152.77-237.42 instead of 152.77-237.42.

12. Line 241, it should be 131.23-187.44 instead of 152.77-237.42.

13. Line 242-245, authors should check the number carefully.

14. Line 245, based on the statistical data, author cannot say that (there is no difference).

15. Line 250-252, there is no comparison between the data in 2018-2019 and 2019-2020. Thus, author cannot say the difference.

**********

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

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29 Sep 2021

Dear Editor,

We appreciate the opportunity to revise the manuscript entitled " Optimizing nitrogen fertilizer amount for best performance and highest economic return of winter wheat under limited irrigation conditions" (Manuscript ID PONE-D-21-25804). We would like to express our sincere thanks to you and the reviewers for your constructive and valuable comments, which are very helpful for improving the manuscript. We have revised the text as described below and also provide point-by-point response to each comment. Changes in the revised manuscript are indicated in yellow font. We sincerely hope that this revised version of the manuscript is qualified for publication in PLOS ONE.

Reviewer #1: Recommendation: Major revision

The survey reports an interesting topic that points out the necessity of nitrogen fertilization optimization in the fields of winter wheat under limited irrigation and aiming a higher economic return. The manuscript is written in a good standard English style and a good statistical approach has been presented. I think the manuscript is of good quality and has a merit to be published in PLOS One when the recommended revision is successfully done. Following, a list of comments that will help authors improve their manuscript.

1. Page 2, line 28: Kindly remove “a” before few.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We removed it at revised manuscript.

2. Page 3, line 40: Kindly add a space between “population” and “[1]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added it at revised manuscript.

3. Page 3, lines 42–43: Kindly provide a reliable source (reference) for this statement.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added reference [3].

[3] Chinese Statistical Bureau. China Statistical Yearbook. Beijing: China Statistics Press; 2020.

4. Page 3, line 44: Kindly remove “mush too frequently” and adjust as follow: “farmers frequently use…”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

5. Page 4, line 58: Kindly adjust as follow: “is reported”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

6. Page 4, line 64: Kindly adjust as follow: “[16,17].

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

7. Page 5, line 67: Kindly define the abbreviation “NUE” at its first mention so that the reader can understand that you’re talking about the nitrogen use efficiency.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We modified to nitrogen use efficiency (NUE).

8. Page 5, line 68: Kindly add a space between “costs” and “[19]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added it at revised manuscript.

9. Page 5, lines 74–78: I support the idea, but try to reformulate as the sentence is too long.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

10. Page 6, lines 83–88: Kindly use the impersonal form for these sentences and avoid the first voice form.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

11. Page 8, line 120: I’m okay with the arrangement; however, we can’t rely on unpublished works as a key reference.

Response: Thank you for your valuable comment. This work has been accepted by Journal of Triticeae Crops. We added this reference at the revised manuscript.

21. Zhang P, Wang HG, Fang et al. Q. Diference of Low-Nitrogen Tolerance in Sedling Quality and Activity of Nitrogen Metabolism Related Key Enzymes betwen Nitrogen Responding Types of Winter Wheat Varieties.2020; Journal of Triticeae Crops.41(9). doi:107.606/ji.sn1.009-1041

12. Page 8, line 124: Kindly replace “In all” by “In sum”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

13. Page 9, lines 128, 130: Kindly adopt an homogeny term: either “growing season” or “cropping season”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "growth" to "growing".

14. Page 9, line 131: Kindly add “respectively” after “11”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added “respectively” after “11”.

15. Page 9, line 134: Kindly even use the atomic form of nitrogen, phosphorus, and potassium or their full name.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "P and K" to " phosphorus and potassium "

16. Page 9, line 143: Kindly put “Leaf area index (LAI)” as a subtitle in italic form.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We put “Leaf area index (LAI)” as a subtitle in italic form. We also made similar changes to the other measurements in the manuscript.

17. Page 9, line 143: Kindly replace “in” by “from”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "in " to " from".

18. Page 10, line 146: Same recommendation as point 13.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "growth" to "growing".

19. Page 10, line 152: Same recommendation as points 13 and 18.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "growth" to "growing".

20. Page 11, line 169–170: Kindly adjust as follow: “fertilizer cost”, and “other costs”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "fertilizer input "to" fertilizer cost" and "other input "to" other cost".

21. Page 12, line 179: I suggest writing the full name of LAI here (leaf area index).

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "LAI" to "Leaf area index".

22. Page 12, line 180: You mean the single effect of location, variety and N rate not their combined effect (interaction); as the interaction of these three factors has no significant effect on the leaf area index in both growing seasons 2018–2019 and 2019–2020.

Response: Thank you for your valuable comment. Yes, it is the single effect of location, variety and N rate on LAI.

23. Page 12, line 183: Kindly add “in both growing seasons” after “N0 treatment”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added “in 2018-2019 growing seasons” after “N0 treatment”.

24. Page 12, lines 186–188: “However… treatments”: Kindly remove this section as discussing non significant results is not important.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We removed this section.

25. Page 12, line 188: Kindly replace “However” by “Moreover”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We replaced “However” by “Moreover”.

26. Page 12, line 189: Kindly adjust as follow: “in Xinle and Zhaoxian experiment locations”. At what growing season?

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the growing season in 2019-2020.

27. Page 12, line 190: In comparison with which N treatment? N0?

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. This is compared with N180 treatment and we added it in the manuscript.

28. Page 12, line 191: Kindly remove “although… N300” for the same reason mentioned in point 24.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We removed this section.

29. Page 13, Table 2: You may point out the interaction between location and variety in both seasons on leaf area index by naming it (L ˟ V) and the interaction between location and N rate in 2019–2020 season (L ˟ N).

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the L×V significantly affected the LAI in 2018–2020, and L×V only affected the LAI in 2019–2020.

30. Page 13, Table 2: Kindly define each abbreviation as a table footnote.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the full names of the abbreviations in the table as footnotes.

31. Page 13, line 195: Kindly remove “Location, variety, and N rate in 2018–2020 (Table 2).”

Response: Thank you for your valuable comment. This is about Location, variety, and N rate significantly affected aboveground biomass in the two growing seasons considered herein. We changed the expression into “Location, variety, and N rate significantly affected aboveground biomass in the two growing seasons considered herein (Table 2).”

32. Page 13, line 195, Figure 3: Kindly correct the order of the locations graphs to be similar in the upper and the lower rows and capitalize the first letter of all locations names.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We corrected the upper and the lower rows and capitalized the first letter of all location names.

33. Page 13, line 196: In which growing season?

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the growing season in 2018-2019.

34. Page 13, line 198: Kindly adjust as follow: “growing season”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed it to “growing season”.

35. Page 13, line 201: Kindly adjust as follow: “N0’s one”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed it into “N0’ treatment”.

36. Page 13, line 202: Kindly add that it was a significant increase in Xinle and Zhaoxian experimental locations.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the significant increase in Xinle and Zhaoxian experimental locations.

37. Page 13, line 203: Same recommendation as in points 13, 18, and 19.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed "growth" to "growing".

38. Page 14, line 204: Kindly mention if it was a significant increase in comparison with the control (N0 treatment), in which season, and by how much.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. This sentence expresses that the nitrogen application rate is increased from N180 to N240 and N300, and the increase in dry matter is mainly due to the increase in vegetative organs.

39. Page 14, line 205: Same recommendation as in point 13, 18, 19, and 37.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed it to “growing season”.

40. Page 14, line 206: Kindly adjust as follow: “in the previous season”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed to “in the previous season”.

41. Page 14, lines 208–210: Kindly mention the growing season.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the growing season in 2018-2019.

42. Page 14, lines 213–214: Actually, you can mention in the previous sentences both seasons instead of splitting the idea into two.

Response: Thank you for your valuable comment that could make a better description. Actually, we did try to merge the two seasons but it became somewhat cumbersome. We will keep improving our English proficiency.

43. Page 14, line 215: Kindly mention the effect of the interactions between location and variety, and location and N rate on the grain yield.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the effect of the interactions between location and variety, and location and N rate on grain yield.

44. Page 14, lines 215–216: Actually, it was the case of all treatments in 2019–2020 season. It looks like it is correlated with environmental changes rather that fertilization effect.

Response: Thank you for your valuable comment. Yes, it is correlated with environmental changes. We explained that in the discussion section.

45. Page 14, lines 217–218: Same recommendation as in points 13, 18, 19, 37, and 39.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed to “growing season”.

46. Page 15, lines 219–227: Kindly mention the effect of the interactions between location and variety, and location and N rate in the growing season 2018–2019 on the harvest index.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the effect of the interactions between location and variety, and location and N rate in the growing season 2018–2019 on the harvest index.

47. Page 15, line 222: Kindly specify the N treatments.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the N treatment.

48. Page 15, lines 223–225: Kindly mention the season and specify the N treatments.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the growing season in 2018-2019.

49. Page 15, lines 225–226: Same recommendation as in point 42.

Response: Thank you for your valuable comment that could make a better description. Actually, we did try to merge the two seasons but it became somewhat cumbersome. We will keep improving our English proficiency.

50. Page 15, line 229: Kindly mention the complete name of NUE in the title.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the complete name of NUE at its first mention.

51. Page 15, lines 230–234: There are plenty of non-discussed results especially concerning the interactions between factors location, variety and N rate and their effect on the studied parameters.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the interactions between factors location, variety and N rate and their effect on the studied parameters in the manuscript.

52. Page 15, line 233: It is the case in both growing seasons.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed to “2018-2020”.

53. Pages 15–16, lines 235–245: It is a reading of the results. Kindly try to compare the results rather than reading them.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We re-analyzed the data in the manuscript.

54. Page 16, lines 246–248: Same recommendation as in points 42 and 49.

Response: Thank you for your valuable comment that could make a better description. Actually, we did try to merge the two seasons but it became somewhat cumbersome. We will keep improving our English proficiency.

55. Pages 16–17, lines 251–252: Good discussion point, but kindly move it to the “Discussion” part.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We moved it to the “Discussion” part in the manuscript.

56. Page 18, line 258: Kindly adjust as follow: “from the two tested winter wheat varieties”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “from the two tested winter wheat varieties” in revised manuscript.

57. Page 18, line 261: Do you mean by “net return” the “economic return” ? Kindly adopt an homogeny term and mention when it increased and decreased.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed “economic return” to “net return”.

58. Page 18, lines 262–270: Kindly follow the recommendation in the next point for this section.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added Duncan test in Table 4 and revised the corresponding results analysis.

59. Page 19, line 272, Table 4: Kindly make the Duncan test for this table to detect which treatments in both varieties and growing seasons give the most significant gross profit and economic return and lower costs. In the current way provided, we cannot rely on the results to judge.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added Duncan test in Table 4 and revised the corresponding results analysis.

60. Page 21, lines 299–302: The sentence is too heavy; kindly reformulate.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We modified the sentence in the manuscript.

61. Page 21, lines 302–303: Kindly adjust as follow: “Altogether, data indicated that…”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “Altogether, data indicated that…”.

62. Page 22, line 311: Kindly adjust as follow: “[25,26]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “[26,27]” (after ref 3 was added).

63. Page 22, line 314: Kindly adjust as follow: “[27,28]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “[28,29]”

64. Page 22, line 316: Kindly adjust as follow: “[29,30]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “[30,31]”

65. Page 23, line 328: Add a space between “[34]” and “showed”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added a space between “[34]” and “showed”.

66. Page 23, lines 334–336: Good discussion of the results !!

Response: Thank you!!!

67. Page 24, lines 339–340: “Improving… resources”: Kindly provide a reliable source (reference) for this statement.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added reference [37].

[37] Guo Y, Chen Y, Searchinger TD, Zhou M, Pan D, et al.Air quality, nitrogen use efficiency and food security in China are improved by cost-effective agricultural nitrogen management.2020;Nature Food.1(10):648-658.doi:10.1038/s43016-020-00162-z

68. Page 24, line 352: Kindly adjust as follow: “[17,36]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “[18, 38]”

69. Page 24, line 352: Kindly adjust as follow: “… NUE; high-N varieties…”

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as follow: “… NUE; high-N varieties…”.

70. Page 24, line 353: Kindly mention the full name at the first mention of a parameter, and follow it by the abbreviation between brackets.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added the “complete name of” in revised manuscript.

71. Page 25, line 359: Kindly adjust as follow: “As it can be seen…”

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “As it can be seen…”.

72. Page 25, lines 359–360: Same recommendation as in points 13, 18, 19, 37, 39, and 45.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed to “growing season”.

73. Page 25, line 361: Kindly adjust as follow: “excessive” or “excess in”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed to “excessive”.

74. Page 25, line 362: Kindly remove “and” before “site-specific”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We removed “and” before “site-specific”.

75. Page 25, line 366: Kindly remove “The” from the paragraph title.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We removed “The” from the paragraph title.

76. Page 25, line 369: Kindly adjust as follow: “[38,39]”.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as “[40,41]”

77. Page 26, line 371: Is it an irrigation or a rainfall ? I guess you mean an annual or a seasonal rainfall. Kindly clarify.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We clarified it is irrigation. Xin [11] showed the yield was the highest when the irrigation rate was 240 mm.

78. Page 26, lines 373–378: Kindly use the impersonal form for these sentences.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We modified the impersonal form for these sentences.

79. Page 26, lines 381–385: The statement is right but is too long. Kindly reformulate.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We checked and modified the sentence.

80. Pages 26–27, lines 385–389: The statement is right but is too long. Kindly reformulate.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We checked and modified the sentence.

81. Page 27, lines 390–391: Kindly adjust as follow: “… management of winter wheat fertilization in the Piedmont plains…”

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We adjusted as follow: “… management of winter wheat fertilization in the Piedmont plains…”

Reviewer #2: The topic is not novel; however, the research data covers the research objectives and the trails at various locations make it valuable.

My only concern is why the author didn't use the control (well-watered conditions) to compare the results under limited water conditions?

I will suggest to compare your results with the control (well-watered conditions) as well.

Response: Thank you for your valuable comment. There are three reasons why there is no comparison between sufficient water conditions and restricted water conditions. The first, Winter wheat is the top irrigated crop in the North China Plain. There is a long history of over-irrigation on wheat using groundwater to achieve maximum yield in this region. However, long-term overexploitation of groundwater for irrigation has resulted in a decline of groundwater in the North China Plain, which has negatively affected the sustainable development of regional agriculture [5]. Therefore, limited irrigation has become a governmental policy encouraging farmers to save water and corresponding techniques have been developed and extended. The second, there are plenty of previous studies with wheat under both well-irrigated and limited water conditions in the piedmont plain. Based on previous studies, we designed and conducted this study under limited water conditions [41]. Finally, the major objective of this study is to identify the optimal nitrogen application rate under limited water conditions in the piedmont plain of the Taihang Mountains.

5. Han S, Tian F, Liu Y, Duan X.Socio-hydrological perspectives of the co-evolution of humans and groundwater in Cangzhou, North China Plain.2017;Hydrology and Earth System Sciences.21(7):3619-3633.doi:10.5194/hess-21-3619-2017

41. Si Z, Zain M, Mehmood F, Wang G, Gao Y, et al.Effects of nitrogen application rate and irrigation regime on growth, yield, and water-nitrogen use efficiency of drip-irrigated winter wheat in the North China Plain.2020;Agricultural Water Management.231(doi:10.1016/j.agwat.2020.106002

Reviewer #3: This study is not unique in itself as similar studies have been conducted in wheat as can been seen from many of the studies that authors have cited. However, this study does provide valuable and useful scientific information that can be helpful in moving toward sustainable wheat framing in the Piedmont plain region, hence I recommend the editor to consider accepting this manuscript for publication with revisions.

I do find the manuscript is well written and is technically sound but there are certain information that I found to be missing or not explained well enough for me to fully understand all the aspects of the material and method section, which I have listed below. Additionally, I was not been able to see any attached file with primary data (e.g grain yield data) used for various analysis which is required as per PLOS Data policy, please request authors to make those data available as supplemental file or deposited to a public repository, if not done so already.

Material and Methods:

Experimental site description:

Line 115: Table 1 is showing NPK measurement prior to 2018 seeding, was similar measurement obtain for 2019 year before seeding? If yes, please add them

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added this information in revised manuscript.

Experimental design

Line 118-124: Please reword and clarify the experimental design description. Was it complete randomized block design? Was the treatment randomized? How many subplots per main plot were there? If the main plot was 50 m. wide and sub-plots were 10m. wide with 1.0 m buffer between subplots then does that mean there were only 4 sub-plots per main plain (ie. Only 4 N treatments per main plot instead of 5 N treatment)

Response: Thank you for your valuable comments based on which we have revised the manuscript accordingly. The experiment adopted a two-factor split plot design at three experimental locations. The variety treatments served as the main plots and N application rate served as the subplots. We added this experimental design description at revised manuscript.

Thank you for your valuable comment. The sub-plots were 10 m long×5 m wide, the 1.0 m buffer is included in a 10-meter long plot. We modified the design description of the experiment in the revised manuscript.

Line 122: What was the reason behind selecting those specific five N fertilizer rates?

Response: Thank you for your valuable comment. We chose these five nitrogen treatments for the following reasons: First, previous studies have found that the optimal nitrogen application rate is between 150-240 kg ha-1 in different sites. Second, in the piedmont plain, farmers usually apply 240-300 kg ha-1 nitrogen on wheat. Therefore, 300 kg ha-1was set as the maximum and 120 kg ha-1 the minimum rate in order to cover all the possibilities.

Line 125: Was commercially available seed used both years for seeding?

Response: Thank you for your valuable comment. Yes, the two varieties are widely adopted by wheat farmers in the Piedmont plain of the Taihang Mountains and are easily available in seed markets.

Line 133: Please provide method of irrigation

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We connected a water meter at the water outlet of the pump, then connect the water pipe to the plots in order to strictly control water quantity at 60 mm. We added this at revised manuscript.

Line 139: Please mention if topdressing of N was done after or before irrigation at the jointing stage.

Response: Thank you for your valuable comment. It was done before irrigation. We added this at revised manuscript.

Line 140: Provide information on which pesticides and herbicides were applied, mode of application and their dosage.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We added this information at revised manuscript.

Measurements and calculations:

Line 153: Was grain moisture estimated after drying and was grain moisture used for adjusting grain yield?

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. Grains were air-dried and weighed after harvest, samples were taken and oven-dried for measuring moisture content. Yields were normalized at 13 % moisture content. In the revised manuscript we added this information.

Data analysis:

Line 173-174: Please provide the ANOVA model. Was year included in the model along with the location? If not, then does ANOVA result differ if year is added to the model.

Response: Thank you for your valuable comment. The location is included in the model, but not the year. Based on your suggestion, we added year to the ANOVA model in revised manuscript.

Results:

Line 187-188: Was no significant differences in LAI for 2019-2020 observed due to higher residual N-P-K content in soil due to application? Was same exact field used in both years of the study?

Response: Thank you for your valuable comments. The same plots were used in both years of the study. During the 2019-2020 growing season, LAI at N180 was significantly higher than N0 treatment. Moreover, in 2019-2020, at the Xinle and Zhaoxian experimental location, when N input increased from N180 to N240, LAI of JM585 increased significantly by 4.34% and 11.95%, respectively. LAI in 2019-2020 was higher than 2018-2019, mainly due to significantly higher rainfall in 2019-2020 (Fig.1).

Discussion:

Line 349-351: any specific environmental reason why grain per spike was reduced in 2019-2020?

Response: Thank you for your valuable comment. This may be because freezing damage during the panicle differentiation period and continuous rain during the flowering period in the 2019–2020 season (Fig.1). This information is in the discussion section.

Line 374-375: Will using available N in soil prior to seeding or fertilizer application improve calculation of required N fertilizer rate for each season more accurately and help improve economic return?

Response: Thank you for your valuable comment. The quantity of nitrogen that a season of wheat crop needs either from soil and fertilizer to achieve satisfactory yield in this region is 180-300kg ha-1. Fertilizer N inputs can be estimated if we know the available nitrogen in the soil. This on-demand input reduces the possibility of nitrogen leaching. However, given the very small farm and field size, it would be too expensive to obtain such data for each field or even each farm.

Reviewer #4: This study tried to find the appropriated nitrogen fertilizer on winter wheat with limited water resources. Authors measured the plant parameters and calculated the economic return. After reading this manuscript, I have several concerns. First, I am not sure the novelty of this manuscript. Second, authors should check the reference format/style carefully. Third, I felt that the cited reference in the main text is not the corrected one. Authors should check for it. In addition, authors used two wheat verities which KN199 has the high NUE and JM585 has the low NUE trait. However, in the table 3, there is no different between KN199 and JM585 on NUE. In addition, authors did not explain the table 2 very well which I think this table has lots of meaning. Besides, I have several comments as follows.

1. Authors should check the usage and grammar. For examples, line 44-45.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We modified the grammar of sentences 44-45.

2. Line 46-49 and 64 and 159, I am not sure that cited references are relevant with the sentence.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We rechecked the correctness of the references.

3. Line 50, reference 78?

Response: Thank you for your valuable comment. These are two references. I forgot to add ", "in the middle, which modified in the revised draft.

4. Line 70-71 and 74-78, what is the reference?

Response: Thank you for your valuable comment. Line 69-71, It's actually the same reference. The language is not well organized. We have revised it in the revised manuscript.

Mehrabi et al. [21] showed that there was no significant difference between 150 and 300 kg N ha-1 in Shiraz, Iran. The optimal N application rate is 150 kg N ha-1.

Line 74-78, This sentence in the manuscript is a summary of the previous part and therefore does not includes references.

5. Line 79, what are the N absorption-related traits?

Response: Thank you for your valuable comment. The N absorption-related traits include grain yield, nitrogen accumulation in grains and plants, and nitrogen use efficiency of wheat.

6. Line 88-90, I am not sure that this manuscript would provide the theoretical basis.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We removed “theoretical basis”.

7. Line 119-121, authors used two different NUE wheat varieties. However, there is no reference on it. As I mentioned earlier, there is no different between KN199 and JM585 on NUE. Authors should explain for that.

Response: Thank you for your valuable comment. We added our published references to the revised manuscript. The following table is the results of the variety test in the paper.

First, a pot-culture variety test was carried out in artificial climate chamber under two nitrogen treatments of N0 and N240. In N0 treatment, only vermiculite (total nitrogen 0.9 g•kg-1) and tap water (total nitrogen 10.2 mg•L-1) provided nitrogen. Therefore, there are differences between artificial climate chamber environment and field environment. Second, the results of the preliminary test were classified into low nitrogen tolerance high yield type and nitrogen sensitive type according to low nitrogen tolerance index. Therefore, in the revised manuscript, ‘high NUE’ and ‘low NUE’ were modified as ‘low nitrogen tolerance and high yield’ and ‘nitrogen sensitivity’. Third, the selection of varieties should first consider low nitrogen tolerance index, wide adoption in the piedmont plain. Therefore, KN199 and JM585 are selected as experimental materials.

8. Line 167, it should be E instead of Eb

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly.

9. In the result section, authors used the combined data in the text but used the separate data in the table. It is hard to judge the number.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We modified it in the revised manuscript.

10. Line 233, it should be 2018-2020.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We changed to 2018-2020.

11. Line 238, it should be 152.77-237.42 instead of 152.77-237.42.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We corrected it in the revised manuscript.

12. Line 241, it should be 131.23-187.44 instead of 152.77-237.42.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We corrected it in the revised manuscript.

13. Line 242-245, authors should check the number carefully.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We made a careful check of the number in the manuscript.

14. Line 245, based on the statistical data, author cannot say that (there is no difference).

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We checked and modified it in the revised manuscript.

15. Line 250-252, there is no comparison between the data in 2018-2019 and 2019-2020. Thus, author cannot say the difference.

Response: Thank you for your valuable comment based on which we have revised the manuscript accordingly. We checked and modified it in the revised manuscript.

More detailed response with tables and figures can be found in the uploaded file "response to reviewers"

Submitted filename: renamed_fa6e1.docx

Click here for additional data file.

18 Oct 2021

PONE-D-21-25804R1Optimizing nitrogen fertilizer amount for best performance and highest economic return of winter wheat under limited irrigation conditionsPLOS ONE

Dear Dr. Liang,

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Academic Editor

PLOS ONE

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Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

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Reviewer #2: Yes

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Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #2: Yes

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Reviewer #1: Recommendation: Minor revision

The revised manuscript’s version represents an improved one compared to the original submission. Authors replied to all my queries in their point-by-point answers. I am glad to say that authors took into consideration the detailed recommendations proposed. Several clarifications of previously ambiguous points were presented. Major improvements in the Results part were noticed; however, minor modifications are required in this section. Overall, the manuscript shows a higher value in its current form. I think the manuscript has a merit to be published in Plos One when the recommended minor revision is successfully done. Following, some comments that will help authors improve more and more their manuscript.

1. Page 9, lines 134–136: Kindly adopt the following manner when stating the methods of fertilization and irrigation: “N fertilizer was spread… a water meter was connected… then the water pipe was connected to the plots…”

2. Page 10, line 144: Kindly adjust as follow: “pesticides”.

3. Page 13, lines 197–199: Kindly adjust the sentence as follow: “Moreover, in 2019-2020, at Xinle and Zhaoxian experimental locations, the LAI of JM585 increased significantly by 4.34% and 11.95%, respectively when the N input increased from N180 to N240”.

4. Page 15, lines 213–214: Kindly adjust the sentence as follow: “aboveground biomass significantly increased in Xinle and Zhaoxian experimental locations.”

5. Page 18, lines 259–260: Kindly adjust the sentence as follow: “total N accumulation significantly increased in the three experimental locations”.

6. Page 20–21, lines 284–292: Kindly compare statistically your results in this section. In other words, compare your results in the following manner: “Net return was significantly the highest (you state the growing season, location and variety) being higher by … (you state the range between the minimum net return and the maximal one). In this manner, you give your valuable study and its results their right and you avoid reading the table. I’m glad seeing the ANOVA test added for gross profit and economic return as recommended and your results look now more valuable. Good work !!

However, kindly add the ANOVA test also for water cost and cost cultivation (all “a” as being the same). Kindly add also the ANOVA test for fertilizer cost in order to detect if there is significance between all N fertilization rates or not, and discuss the obtained results.

7. Page 26, lines 367–368: Kindly adjust the sentence as follow: “and when the latter exceeded 240 kg ha-1, they remained increased in a non-significant way”.

8. Page 26, line 373: Kindly adjust the sentence as follow: “This may be due to frost damage…”

Reviewer #2: (No Response)

**********

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Reviewer #2: No

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8 Nov 2021

Reviewer #1

We very much appreciate your time and kind detailed comments on the manuscript. Revises have been done accordingly.

1. Page 9, lines 134–136: Kindly adopt the following manner when stating the methods of fertilization and irrigation: “N fertilizer was spread… a water meter was connected… then the water pipe was connected to the plots…”

Response: Revised as advised.

2. Page 10, line 144: Kindly adjust as follow: “pesticides”.

Response: Revised as advised.

3. Page 13, lines 197–199: Kindly adjust the sentence as follow: “Moreover, in 2019-2020, at Xinle and Zhaoxian experimental locations, the LAI of JM585 increased significantly by 4.34% and 11.95%, respectively when the N input increased from N180 to N240”.

Response: Revised as advised.

4. Page 15, lines 213–214: Kindly adjust the sentence as follow: “aboveground biomass significantly increased in Xinle and Zhaoxian experimental locations.”

Response: Revised as advised.

5. Page 18, lines 259–260: Kindly adjust the sentence as follow: “total N accumulation significantly increased in the three experimental locations”.

Response: Revised as advised.

6. Page 20–21, lines 284–292: Kindly compare statistically your results in this section. In other words, compare your results in the following manner: “Net return was significantly the highest (you state the growing season, location and variety) being higher by … (you state the range between the minimum net return and the maximal one). In this manner, you give your valuable study and its results their right and you avoid reading the table. I’m glad seeing the ANOVA test added for gross profit and economic return as recommended and your results look now more valuable. Good work !!

However, kindly add the ANOVA test also for water cost and cost cultivation (all “a” as being the same). Kindly add also the ANOVA test for fertilizer cost in order to detect if there is significance between all N fertilization rates or not, and discuss the obtained results.

Response: Revised as advised.

7. Page 26, lines 367–368: Kindly adjust the sentence as follow: “and when the latter exceeded 240 kg ha-1, they remained increased in a non-significant way”.

Response: Revised as advised.

8. Page 26, line 373: Kindly adjust the sentence as follow: “This may be due to frost damage…”

Response: Revised as advised.

In addition,

1. In economic analysis, “benefit” was replaced with “return” and the changes were marked in yellow font.

2. The manuscript format was modified to meet the requirements of PLOS ONE. We added figure captions appear directly after the paragraph in which they are first cited in the manuscript.

3. Table and figure captions were revised.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

9 Nov 2021

Optimizing nitrogen fertilizer amount for best performance and highest economic return of winter wheat under limited irrigation conditions

PONE-D-21-25804R2

Dear Dr. Liang,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Vassilis G. Aschonitis

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

16 Nov 2021

PONE-D-21-25804R2

Optimizing nitrogen fertilizer amount for best performance and highest economic return of winter wheat under limited irrigation conditions

Dear Dr. Liang:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Vassilis G. Aschonitis

Academic Editor

PLOS ONE