Evaluation of residue management practices on barley residue decomposition.

Optimizing barley (hordeum vulgare L.) production in Idaho and other parts of the Pacific Northwest (PNW) should focus on farm resource management. The effect of post-harvest residue management on barley residue decomposition has not been adequately studied. Thus, the objective of this study was to determine the effect of residue placement (surface vs. incorporated), residue size (chopped vs. ground-sieved) and soil type (sand and sandy loam) on barley residue decomposition. A 50-day(d) laboratory incubation experiment was conducted at a temperature of 25°C at the Aberdeen Research and Extension Center, Aberdeen, Idaho, USA. Following the study, a Markov-Chain Monte Carlo (MCMC) modeling approach was applied to investigate the first-order decay kinetics of barley residue. An accelerated initial flush of residue carbon(C)-mineralization was measured for the sieved (Day 1) compared to chopped (Day 3 to 5) residues for both surface incorporated applications. The highest evolution of carbon dioxide (CO2)-C of 8.3 g kg-1 dry residue was observed on Day 1 from the incorporated-sieved application for both soils. The highest and lowest amount of cumulative CO2-C released and percentage residue decomposed over 50-d was observed for surface-chopped (107 g kg-1 dry residue and 27%, respectively) and incorporated-sieved (69 g kg-1 dry residue and 18%, respectively) residues, respectively. There were no significant differences in C-mineralization from barley residue based on soil type or its interactions with residue placement and size (p >0.05). The largest decay constant k of 0.0083 d-1 was calculated for surface-chopped residue where the predicted half-life was 80 d, which did not differ from surface sieved or incorporated chopped. In contrast, incorporated-sieved treatments only resulted in a k of 0.0054 d-1 and would need an additional 48 d to decompose 50% of the residue. Future residue decomposition studies under field conditions are warranted to verify the residue C-mineralization and its impact on residue management.

Introduction

Barley (hordeum vulgare L.) is an important cereal crop grown worldwide with a production of 142 million metric ton (MMT) in 2018–2019 crop year [1] with a total production of 3.09 MMT covering nearly 790,000 hectares in the USA [2]. Idaho represents one-third of total US barley production which accounted for ~1.1 MMT of barley production annually over 200,000 hectares [3]. Globally, fifty-nine countries import US barley largely for use in the beer industry [4]. Common practices of barley production involve harvesting of barley grains and afterwards, barley residue either remains in the field or is bailed and removed. Remaining residue in the field is an important source of soil organic carbon (SOC) [5]. In semi-arid regions of the Pacific Northwest (PNW) and western United States (for example, Idaho), soils are susceptible to soil organic matter (SOM) loss, thus, SOC levels may be low [6]. In addition to building SOC, residues reduce soil erosion, retain and recycle plant nutrients in the soil, and provide required energy for soil microbial processes [5,7-10].

Carbon to nitrogen (C:N) ratio is a contributing factor in determining the residue decomposition as affected by existing soil microbial communities. Sustainable nutrient management for barley production in Idaho and other parts of PNW are based on the knowledge of how residues affect the cycling of C and N, and the availability of crop nutrients in the soil. Critical factors in barley residue breakdown include soil type, tillage management, and management of the post-harvest barley residue. Barley residue can vary in its C: N ratio (54 to 80:1) depending on the growing condition and location [11]. Further, tillage operations in Idaho and other PNW regions vary widely from no-tillage, direct-seed operations to intensive conventional tillage operations that can subsequently affect residue breakdown. Conventional tillage incorporates residues, surface soil is turned over into the sub-surface soil [12] and the rhizosphere becomes more stable for residue breakdown [13,14]. Incorporation of crop residues in a wheat (triticum aestivum L.) field in Oregon (~PNW) over a 26-month field study showed that the net mineralization was typically greater when residues were plowed into the soil (total 85%) than when residue was left on the soil surface (25% and 31%, respectively) [15] over the course of experiment, and provided greater nutrients for subsequent crops. However, the opposite trend of increased breakdown of corn (zea mays L.) residue occurred when residues remained on the surface of the soils (~no tillage) compared to incorporated in recent studies in Iowa [10,16]. The method of residue placement (surface vs. incorporated) and consequently, the contact between the soil and the residue have had conflicting results indicating the effects of other confounding factors (i.e. residue properties, native microbial communities) on the residue breakdown rate in previous research. Residue particle size of barley residue in the field as part of management practices could aid either faster or slower breakdown by various mechanisms [14]. In general, finer sized residue would allow more surface area for microbial activities and nutrient and/or water retention in the surface of the soil to facilitate residue breakdown [5,10]. In contrast, the application of residues in coarser chopped conditions would result in reduced surface contact between the soil and barley straw which has been claim to slow down the rate of residue breakdown by microbial decomposers [17]. Thus, smaller sized residues may favor C-mineralization and residue breakdown in barley by either increased microbial activities particularly by increasing soil temperature [15] or by filling the soil pore-space in higher magnitude which facilitate decomposition [5]. Alternately, an increased C-mineralization (~8% more) from small-grain residues with the application of coarser-sized residue as compared to fine-sized residue during a 65-d laboratory incubation study at 25°C were also reported [11].

Residue breakdown is also affected by soil type and associated soil properties (for example, organo-mineral interactions) specific to a region as well as interactions that occur in the rhizosphere among the plant, soil microbial organisms, and soil flora and fauna [18]. Barley residue decomposition was affected by soil texture, for example reduced breakdown with a greater adsorption of organic N and C (stronger organo-mineral bonding) in clay soils [18] vs. greater breakdown with a reduced rate of SOM decomposition in highly aerated sandy loam soils [13,19]. Soil type has been reported to have a more prominent effect on the rate of decomposition than the residue N content [13]. Recently, no differences in decomposition rate of corn residues over an extended period of time were observed in a silty-clay loam soil in Iowa after addition of N to the soil [5]. Other associated soil properties such as soil moisture in addition to soil type significantly influences the breakdown of the residue [13].

The effects of various factors in the process of barley residue decomposition as mentioned above showed mixed results based on the literature available. Additionally, the effect of post-harvest management practices of barley residues in irrigated production systems on C-mineralization needs to be quantified and understood to ensure sustainable barley production. Achieving sustainable barley production through understanding the dynamics of residue carbon breakdown and residue CO2-C loss for subsequent crop production is the focus of this study. Our objectives were (i) to determine the effect of residue placement (surface vs. incorporated), residue size (chopped vs. sieved-ground) and soil type (sand and sandy loam) on barley residue decomposition rate, (ii) to quantify the barley residue decomposition with a modeling approach to guide proper resource management for barley and cereal production.

Materials and methods

Site description

Two soil samples (with four field replicates) were collected from a common soil series (Declo Loam; Coarse-loamy, mixed, superactive, mesic Xeric Haplocalcids) in southeastern Idaho [20,21] (Table 1). While both the soil samples were from the same soil series they differed in textural class (sand versus sandy loam) [21]. Before soil sample collection, the lands were cultivated with barley (Hordeum vulgare, L.). The average soil organic carbon content measured by loss on ignition (LOI) in the study region was 2 g kg-1 [22]. Within a sampling location, an approximately 0.1 ha area was sampled by collecting and compositing 20 sub-samples using a 7.6-cm bucket auger from depths of 0- to 15- cm. Tillage operations can be highly varied in Idaho and other western States. The depth of 0–15 cm is an approximation for shallow tillage operations that are conducted using disc plows [5]. Collected soil samples were dried in a forced-convection oven at 40°C, and subsequently crushed and passed through a 2-mm sieve.

Table 1

Basic properties of soil used in the laboratory incubation study conducted at the Aberdeen Research and Extension Center, Aberdeen, ID, USA.

Properties	Sand	Sandy Loam
Sand (g kg-1)	898 (±4.9)†	661 (±5.6)
Silt (g kg-1)	57 (±15.1)	239 (±5.6)
Clay (g kg-1)	45 (±11.0)	100 (±0.02)
pH	8.1 (±0.01)	8.4 (±0.02)
EC (μS cm-1)	102 (±3.04)	148 (±5.9)
SOM (g kg-1)	9 (±0.6)	15 (±0.6)
Total N (g kg-1)	0.46 (±0.01)	0.69 (±0.02)

†values in the parenthesis indicate standard errors.

EC; electrical conductivity, SOM; soil organic matter; N; Nitrogen.

Soil description

Soil particle size analysis was performed using the hydrometer method (Table 1) [23]. Soil pH and electrical conductivity (EC) were determined potentiometrically using a 1:1 soil to deionized water ratio [23] using a soil pH meter (Orion StarAM A215 pH/Conductivity Benchtop Multiparameter meter, Thermo Fisher Scientific Inc., Waltham, MA, USA). The loss on ignition (LOI) analysis was conducted on samples where 10 g of the sample was dried at 105 °C for 2 h and placed in a desiccator for 1 h. Samples were then combusted in a muffle furnace at 360 °C for 2 h, dried for 1 h at 105 °C and equilibrated in a desiccator for 1 h. The SOM content was determined by LOI based on the difference in initial and final weights [23,24]. Total nitrogen (TN) was measured by high-temperature combustion using a VarioMax CN analyzer (Elementar Americas, Inc. Mt Laurel, NJ).

Residue description

The residue used in the experiment was obtained from a common malt barley cultivar (Harrington) which is grown for malt production in the study region [25]. Residues were collected in 2017 after harvest of a barley crop grown under standard production practices at the Aberdeen Research and Extension Center, Aberdeen, ID [26]. An estimated dry matter production for barley excluding grain (~8000 kg ha-1) was chosen as the rate of residue added for the study [27]. The residue was characterized for cellulose, hemicellulose, lignin and ash content. The cellulose–Acid Detergent Fiber (ADF; ash-free) content were determined using the method of ‘Fiber (Acid Detergent) and Lignin in Animal Feed: Section: 973.18’ [28] with a modification of using Whatman 934-AH glass micro-fiber filters with a capacity of 1.5μm particle retention instead of using the fritted glass crucible. The hemicellulose-Neutral Detergent Fiber (NDF; ash-free) content was determined using the method described by [29]. Lignin, a critical factor influencing digestibility of the plant cell wall, was analyzed by the method outlined by [30]. Followed by fiber (ADF and NDF) and lignin analyses, the ash content in the residue samples were analyzed using the methods detailed in the ‘Ash of Animal Feed; section: 942.05’ [28].

Laboratory incubation experimental approach

A 50-d laboratory incubation experiment was conducted at a constant temperature (25°C) and moisture (60% water-filled pore space). Although we did not mimic the field condition, the experimental set-up we used allowed us to compare our findings with other microcosm studies focused on residue decomposition [5,31]. The experiment was arranged as a randomized complete block (RCB) design with two types of residue placement (surface vs. incorporated), two residue sizes (chopped vs. sieved), and two soil textures (sand vs. sandy loam) with four replicates for each. Barley residue (4.1 g) and soil (100 g) were placed in a Mason jar. A 500 mL capacity wide-mouth mason jars with an 86 mm dome lid modified to support a 60-mm petri dish [32] were used in this study. The inner diameter width 76 cm, outer diameter width of 82 cm, and height of 113cm. (The inner diameter width 76 cm, outer diameter width of 82 cm, and height of 113cm). Surface applied residue was evenly spread on the surface of the soil and incorporated residue was thoroughly mixed into the soils. Residue size included chopped (~7-cm) and sieved (ground to pass a 2-mm sieve) treatments.

The study was conducted based on incubation methods described by [5]. A 60 mm Petri-dish containing 5 mL of 1 M NaOH solution was placed in each Mason jar to capture CO2 evolved from the soil-residue mixture during the incubation. The amount of CO2 evolved from residue decomposition trapped in NaOH solution at specific time intervals was determined by titration immediately by adding 5 mL of 2 M barium chloride (BaCl2) solution and 2 to 3 drops of phenolphthalein indicator to each petri-dish and pH endpoint titrating with 1 M HCl solution using a digital auto-titrator (848 Titrino Plus, Metrohm Lts., Herisau, Switzerland) until the pH endpoint was reached. A new petri dish with 1 M NaOH was used at each time interval after taking the previous petri-dish from individual mason jars. Three mason jars containing only soil of each type without any residue added were used as a control to monitor CO2 release from the soil. Additionally, three empty Mason jars without any soil or residue were included as an experimental control (control Mason jars) which were used to calculate the total CO2-C evolved from each treatment in the headspace (See the calculation section in supporting information).

Mason jars were weighed initially with the soil-residue mixtures and each Mason jar was weighed each time the petri-dish was changed for taking the reading. Additionally, the amount of CO2 evolved from each jar via absorption by sodium hydroxide (NaOH) in the petri-dish was taken into account for weight reduction in each jar. To maintain the constant soil moisture, the overall reduction in the weight of the Mason jar was compensated by sprinkling the exact amount of deionized water in each jar during the entire experiment.

Calculations

First-order decay constant and decay time. Once the endpoint in titration was achieved, the amount of CO2 retained in each petri-dish was determined by using the formula described by [33]. The initial rate of residue decomposition or decay constant (k) was calculated using a simple first-order decay function (Eq 1, see [34] for more details) within a Bayesian Markov Chain Monte Carlo (MCMC) framework (Eq 1). Where,

Ct = C content at time t (day)

C0 = initial C content (mg)

k = first-order rate constant (day-1)

t = time (day)

The joint probability distribution of the Bayesian MCMC model used is as follows (Eq 2): Where, C0, Ct, and σp (aka process model uncertainty) followed uninformed gamma prior distributions and k followed an uninformed beta prior distribution.

We implemented the MCMC method using the rjags package in R (version 3.4.3). Posterior distributions of k were estimated with 10,000 model iterations after discarding the first 5000 runs. Model convergence was diagnosed with the Gelman–Rubin diagnostics [35] and model suitability were determined with a Bayesian p value.

Using decay constant (k) values, time required for 50, 75, and 99% residue C-mineralization from barley residue was estimated (Eqs 3, 4 & 5) as follows:

Calculations of CO2-C decomposed

Once the endpoint in titration was achieved, the amount of CO2 retained in each petri-dish was determined by using the following formula (33; Eq 6). Where,

X = volume of acid needed to titrate the petri-dish solution from the Mason jars with soil sample only to the end point i.e. ‘blank’, (mL)

Y = volume of the acid needed to titrate the petri-dish solution from the Mason jars with soil-residue sample to the end point i.e. ‘blank’, (mL)

N = normality of the acid, (mL-1)

W = the equivalent weight of C in CO2; W would be 6 if data is to be expressed in terms of C, (mg CO2-C).

Statistical analyses

Treatment effects on cumulative C-mineralization rate (residue breakdown), the percentage of residue decomposed, first-order decay kinetic parameters such as decay constant and the number of days to decompose the residue was analyzed using an analysis of variance (ANOVA). Statistical analyses were performed in JMP 13.0 (SAS Institute, Cary, NC, USA). Tukey’s multiple comparison procedures, as well as the corresponding letter grouping method, was used to separate the treatment means as post-hoc multiple comparisons where p≤ 0.05.

Results

Characterization of residue

The barley residue had a TN of 4.6 g kg-1 dry residue whereas, the TC was 387.5 g kg-1 dry residue which gave a C:N ratio of 85:1 (Table 2). In general, a higher C:N ratio is expected for barley residue as a cereal crop compared to broadleaf crops which matched our data. The cellulose and hemicellulose content of the barley residue were 447 and 670 g kg-1 dry residue, respectively, where the lignin content was 64 g kg-1 dry residue.

Table 2

Basic properties of barley residue samples used in laboratory incubation study conducted at the Aberdeen Research and Extension Center, Aberdeen, ID, USA.

Properties	Unit (on dry residue weight basis)	Average
Total N	g kg-1	4.6(±0.9) ††
Total C	g kg-1	387.5(±4.9)
C:N	-	85(±1.7)
Cellulose	g kg-1	447(±16)
Hemicellulose	g kg-1	670(±16)
Lignin	g kg-1	64(±2.0)
Ash	g kg-1	120(±8.0)

†barley cultivar: Harrington

††values in the parenthesis indicate standard errors.

N; nitrogen, C; carbon, C: N; carbon to nitrogen ratio.

Daily and cumulative decomposition of barley residues

The daily response of the C-mineralization rate (as measured by CO2 released) with respect to the residue placement (surface vs. incorporated) and residue size (chopped vs. sieved) were variable. Both the soils showed a similar pattern in daily residue decomposition breakdown (Fig 1A and 1B). In general, an initial C-mineralization flush by individual treatment combinations for both soils ranged between 2 to 5 days (Fig 1A and 1B). However, the accelerated initial mineralization rate at the beginning of the experiment was obtained quicker and at a greater magnitude for the sieved residues (at Day 1 to 2) than chopped (at Day 3 to 5) for both surface application and incorporation. The highest evolution of CO2-C of 8.3 g kg-1 dry residue was observed on Day 1 from the incorporated-sieved application for both the soils (Fig 1A and 1B). The cumulative decomposition of barley residue increased until it reached a plateau for both the soils (Fig 2A and 2B. The pattern of this cumulative decomposition over a period of 50-d followed a similar trend for all the treatment combination as daily C-mineralization rate for both soils (Fig 2A and 2B).

Fig 1

Amount of CO2-C released (rate as calculated per kg of residues) on daily basis for barley residue decomposition under residue placement and residue size treatment combinations for A) sandy and B) sandy loam soils in 50-days’ laboratory incubation study at 25°C.

Fig 2

Amount of cumulative CO2-C released (rate as calculated per kg of residues) for barley residue decomposition under residue placement and residue size treatment combinations for A) sandy and B) sandy loam soils in 50-days’ laboratory incubation study at 25°C.

Effects of residue management on barley residue decomposition

The ANOVA results indicated that residue placement, residue size, and their interaction had a significant effect on cumulative CO2-C release, percent of residue decomposed, and calculated residue decay constant over a period of 50-d (p<0.001) (Table 3). There were no significant effects of soil type (sandy vs. sandy loam) on any of the measured output in our study although two soils were textually and chemically different. Similarly, the higher-order interaction among residue placement, residue size, and soil types were all non-significant for cumulative CO2-C release, decay constant, and percent of residue decomposed. Averaged across soil types, the highest (107 g kg-1 dry residue) and lowest (69 g kg-1 dry residue) amount of cumulative CO2-C released over 50-d were from surface-chopped and incorporated-sieved treatments, respectively (Fig 3). However, there were no significant differences in CO2-C release between surface-sieved (101 g kg-1 dry residue) and incorporated-chopped (98 g kg-1 dry residue) treatments (Fig 3A).

Table 3

Analysis of variance (ANOVA) P values for cumulative decomposition rate, percentage residue decomposed, decay constant (k) and associated parameters over 50-days’ laboratory incubation study for barley residue decomposition conducted at the Aberdeen Research and Extension Center, Aberdeen, ID, USA.

Sources of variation	Cumulative CO2-C decomposition	Residue decomposed	Decay constant (k)	t(0.5)	t(0.75)	t(0.99)
Residue placement (RP)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Residue size (RS)	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Soil type (ST)	0.09	0.09	0.34	0.07	0.23	0.09
RP x RS	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
RP x ST	0.17	0.16	0.07	0.18	0.09	0.20
RS x ST	0.10	0.10	0.66	0.13	0.72	0.12
RP x RS x ST	0.24	0.27	0.09	0.35	0.10	0.41

Fig 3

Effects of residue placement and residue size on A) cumulative CO2-C released (rate as calculated per kg of residues) and B) percentage of residue decomposed for barley residue decomposition averaged over two soils types in 50-days’ laboratory incubation study at 25 °C.

As expected, the percentage of residue decomposed followed the same pattern as the C-mineralization rate for various treatment combinations (Fig 3B). When averaged across soil types, the highest and lowest amount of residue decomposed over a 50-d decomposition period was measured in surface-chopped (27%) and incorporated-sieved (18%), respectively (Fig 3B). The surface-sieved vs. incorporated-chopped treatments showed no significant differences in their mean values of percentage of residue decomposed (25% for both the treatments).

First order decay kinetics and half-life of barley residues

The ANOVA results indicated that the residue placement, residue size, and their interaction have a significant effect on the decay constants, t(0.5), t(0.75) and t(0.99) residue C-mineralized estimated (p<0.001) (Table 3). There were no significant effects of soil types on any of the calculated parameters. The first-order decay kinetics parameters calculated using MCMC modeling approach were well aligned with model parameter uncertainty. In addition the sensitivity analyses were performed and model predictions were satisfactory as shown in (S1 Fig). The highest k value (0.0083±0.0005 d-1; R2 = 0.997) with a predicted residue half-life to mineralize barley residue (t(0.5)) of about 80+3 d was obtained for surface-chopped applied residues when averaged across soils (Table 4). This value did not statistically differ from surface-sieved (k = 0.0080±0.0006 d-1; R2 = 0.993) and incorporated-chopped (k = 0.0079±0.0007 d-1; R2 = 0.993) treatment combinations. The lowest k value (0.0054±0.0005 d-1; R2 = 0.988) was calculated for incorporated-sieved residues when averaged across soils types (Table 4). Incorporated-sieved treatments would require the maximum duration to decompose 50% of the residue (~128 d). The k and t(0.5) values of incorporated-sieved residues differ significantly from other three treatment combinations (i.e. surface-chopped, surface-sieved, and incorporated-chopped treatments). Similar trends were found for t(0.75) and t(0.99) for all the treatment combinations (Table 4).

Table 4

First-order decay model parameters for sequential barley residue decomposition under residue placement and residue size treatment combinations averaged across soil types in 50-days’ laboratory incubation study at 25°C.

Residue placement	Residue size	Mean k-values (d-1) (±95% CI)	Goodness of fit statistics (R2)	Mean t(0.5) (±95% CI)	Mean t(0.75) (±95% CI)	Mean t(0.99) (±95% CI)
Surface	Chopped	0.0083a††(±0.0005)	0.997	80b (±3)	165b (±6)	552b (±40)
	Sieved	0.0080a (±0.0006)	0.993	86b (±5)	171b (±10)	569b (±34)
Incorporated	Chopped	0.0079a (±0.0007)	0.993	88b (±8)	175b (±16)	582b (±53)
	Sieved	0.0054b (±0.0005)	0.988	128a (±13)	257a (±22)	853a (±72)

Different letters for each parameter indicate significant differences between residue management, residue size and their interaction, as compared using Tukey’s Protected honest significant difference (HSD) test at p<0.05, respectively. CI; confidence interval; SL; sandy loam soils.

Discussions

The initial flush from C-mineralization for crop residues under laboratory, greenhouse, or field conditions is usually significantly different from the decomposition rate for the rest of the experimental duration [5,10,11,36]. This is because the maximum decomposition rate occurs in this period due to soil nutrient dynamics, moisture availability (and soil aeration status), and labile organic material that first undergo decomposition by triggering microbial activity. The initial higher flush of barley straw breakdown or greater C-mineralization in our study was also observed in other cereal residue decomposition studies [5,10,11,31,36].

The rapid rate of C-mineralization from residues in the initial phase (Day 1 to 2) was possibly due to a higher surface area and associated stimulation in microbial activities in sieved compared to the chopped residues (Fig 1A and 1B). Our results were in agreement with the findings by [36] who showed the greater mineralization by finer sized wheat straw at the initial phase of residue addition (~1–2 days). After the initial transitory phase, the decreased rate of decomposition for surface-sieved residues might be attributed due to the exhaustion of those easily decomposable C sources and possibly due to the residue-mineral interaction later in the experiment [11]. They noted a similar pattern of decomposition where finer sized rye and wheat residues (0.05- to 1-mm sieved particles of residues) produced less CO2-C than larger sized residues (7-mm) in their experiment when mixed with a silty soil at 25°C in a laboratory incubation.

Enhancing the physical contact between crop residues and soil minerals may have further promoted the residue-mineral interactions, resulting in lower cumulative decomposition in the incorporated treatment than the surface treatment [5,11,37]. The rate of residue decomposition, subsequently, declined after approximately Day 5 regardless of treatment combination for both the soils (Fig 1A and 1B). This pattern followed other studies conducted either in field or laboratory with residues from different cereal crops grown in variable soil types [5,10,11,31]. These decreased rates of CO2 evolution from the residues were followed by a slight increase at Day 8 to 9 for all treatments and again decreased until the end of the experiment. The difference in the decomposition rate among treatment combinations remained consistent except for the surface applied sieved residues which had a peak CO2 rate of 5.8 g kg-1 dry residue on Day 11.

Effect of residue management on barley residue decomposition

Our study indicated that during the duration of the experiment the application of chopped residues on the surface of soils (Fig 3A and 3B) representing a no-tillage condition, promotes residue breakdown slightly faster than incorporated-chopped residue representing the conventional tillage practices and is supported by other findings where a similar pattern was observed [10,38]. Although no microbial community analyses were performed under the present study, we visually observed greater fungal growth on the surface applied residue than incorporated. Thus, different microbial interactions with the soil-residue mixture could be expected with incorporated residues as compared to surface residues. Our findings were also supported by [38], that reported a reduced rate of residue breakdown due to the lack of fungal communities when sorghum (sorghum bicolor L.) residues were incorporated as compared to the residue that was left on the surface of the soil. The results from our study could have value in promoting conservation practices and preserving residues on the soil surface rather than incorporation via conventional tillage practices in the PNW where soil erosion is a reasonable concern for soil and water quality maintenance. This is worth mentioning that sieved residue had higher decomposition rate than chopped residue in the initial stage, which may be due to higher surface area and thus more access for microbial reaction. However, the trend in the later stage of decomposition phase (after ~Day 5) reverse.

Our results provide evidence that the physical size of the barley residue (chopped vs. sieved) had a significant effect on the C-mineralization rate. For example, the surface-chopped and incorporated-chopped residue produced significantly greater CO2-C than surface-sieved and incorporated-sieved residues, respectively (Fig 3A). The surface-chopped residues were more conducive to decomposition (decomposed 27% of applied residue) and produced greater amount of CO2-C than surface-sieved residues over a 50-d period. The sieved residues may have been stabilized by association with soil mineral surfaces, rendering them inaccessible to decomposition as compared to the sieved residues [11,37,39]. Differences were observed in the initial decomposition rate through day-5 which afterwards disappeared. In addition, the amount of C that were not mineralized in the initial period decomposition (~ 5 days) remained quite stabilized in the soil for longer duration after initial flush of CO2-C release. Thus, the fine-sized grinding of residue provided no advantage over the chopping of residues in terms N release (and subsequent crop uptake) from decomposing residue for sustainable agricultural production [11]. We conclude that extreme mulching practices did not have an advantage in terms of C-mineralization and residue breakdown. Our results were supported by other field studies which showed greater cereal reside decomposition was observed in surface-chopped applied residue by maintaining a larger particle size of residues in the field [10,11,31,36].

Our study did not show any significant differences in the cumulative CO2-C release by the two soils (Table 3). These two soils possess similar soil texture, however, these soils were selected as they are representative of the relatively similar soils found in the study region (south-eastern Idaho) (Table 1). Due to their similar soil texture, the CO2-C mineralized from both the soils irrespective of the type of residue added were similar. This might be due to the fact that both soils have lower clay or organic matter content (0.9–1.5%) which has influenced the aeration and moisture status of the soil-residue mixtures [18]. Further, the presence of clay mineral (organo-clay mineral association) controls the magnitude of residue-mineral interactions which was not significantly different between these two soil types as both are relatively low in clay content. Additionally, other associated soil biological factors such as SOM did not vary widely which could have possibly made a difference in microbial populations and nutrient dynamics in these two soil types, and thus, differences in CO2-C release from the soils.

Prediction of barley residue decomposition: Modeling approach

The first-order kinetic model parameters were calculated using the MCMC approach described the C-mineralization during the initial stages of barley residue decomposition (Table 4). Our study indicated that the highest and lowest rate of both C-mineralization and percentage of residue decomposed were reported from surface-chopped and incorporated-sieved residues, respectively. This resulted in higher values of k for surface-chopped residues, which did not differ from surface sieved or incorporated chopped, as compared to the lowest k values in incorporated-sieved residues. It implies that the incorporated-sieved residues would take the longest time to decompose 50%, 75%, and 99% of the residues and only 18% of the residues were decomposed under the course of the experiment (50-d) (Table 4). Further microbial community analyses which estimates the population of heterotrophic microorganisms (e.g. fungi, different bacterial phylum, etc.) and information regarding their community composition and structure (e.g. bacterial:fungal ratio determination using Phospholipid-derived fatty acid; PLFAs [40]) across different soil types might explain the variability in k values for various treatment combinations.

The implication of the calculation of decay constant and predicted number of days to decompose a certain percentage of residue have serious consequences on residues remaining in the field. For example, if we aim towards a residue management practice that will facilitate residue decomposition and increase the nutrient availability especially mineral C and N to subsequent crops. However, faster residue mineralization might have a priming effects by increasing the decomposition of native organic matter, thus, reducing the soil's organic matter buildup [41] The enhancement in the native SOM by addition of fresh substrate in the initial stage of this study was driven by a temporary increase in the biomass feeding on the more labile soil C [11]. However, to capture the priming effect, it would require labeling of either soil or the residue, which warrant a potential future study.

The most critical factor in soil C sequestration in this type of study is the enhanced physical contact between decomposing residues and soil minerals or organic-mineral contacts. For example, surface application of crop residues can reduce soil disturbance and increase SOC content but heavily respired by microbial communities, thus potentially contributing most to the formation of SOC [11]. On the other hand, incorporated residue in the soil can potentially enhance the SOC stock by having limited physical contact between residues and soil minerals [37]. Thus, the difference between the C stabilized from residue incorporation and C mineralized from tillage determines the net effect on the SOC stocks. These results can suggest that enhancement of physical contact/protection between organic materials and soil minerals (and/or organo-mineral interaction) may promote C stabilization as determined by various treatment combinations in our study. To that end, we need to realize that two potential strategies need to be considered for soil C stock in agricultural soils such as no-tillage agricultural practices and crop residue or manure incorporation. Therefore, a better understanding of mechanisms that control organic-mineral interactions is needed for us to ameliorate current agricultural practices that facilitate SOC retention and sequestration.

Conclusions

Our study considered some critical factors in barley post-harvest residue breakdown such as residue application methods, residue, size and soil types. This 50-d laboratory incubation study resulted in the C-mineralization rate of barley residues in the following order: surface-chopped>surface-sieved = incorporated-chopped>incorporated-sieved. Results indicated the magnitude of mineralizable C released from residue decomposition under different treatments were variable. Additionally, the effects of no-till and conventional tillage as represented by surface vs. incorporated residue application methods on barley residue decomposition under laboratory conditions were documented. Thus, the relevance and application of our findings would be critical in providing practical information on residue decomposition of a barley crop for various residue application methods. Although our study indicated that it is possible to obtain a greater residue C-mineralization for surface-chopped residue than surface sieved under controlled laboratory conditions, it may impact SOM build up. However, barley residue decomposition study under field conditions along with microcosm experimentation in laboratory, and measured at varying times during the crop growing season is warranted to achieve a definite conclusion. Besides, the inclusion of residues in soils with varying SOM content and/or texture profile (e.g. fine- vs. coarse-textured) would be helpful to achieve a definite conclusion. As recommendations are developed, they should consider the advantages and disadvantages of faster or slower residue breakdown under field condition, which is location- and soil- specific. Thus, it would be interesting to see if various residue management practices have coupling effects on soil C (soil C formation and loss) and N cycle (nitrification and denitrification) processes.

(XLSX)

Click here for additional data file.

Calibration and prediction of the first order decay constant for residue decomposed under barley residue decomposition study conducted in laboratory at 25°C.

(DOCX)

Click here for additional data file.

4 Feb 2020

PONE-D-19-33598

Evaluation of residue management practices on barley residue decomposition

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Reviewer #1: The study investigates the effect of three different factors, a. residue placement (surface vs. incorporated), b. residue size (chopped vs. ground sieved) and c. soil type (sand and sandy loam) on barley residue decomposition rates by using a specific modeling approach.

In general, introduction section covers sufficient the background knowledge regarding the objectives of the study. Moreover materials and methods are clear and easily captured by the reader. In this line properties of soils and residue used are properly described by the study. Results and discussion are also adequate. However, more detailed discussion could be strengthen the results and provide the basis for the future work that is needed.

Overall, the manuscript is interesting and provide possible options regarding the effects of residue management in decomposition rates of residues left from barley cultivations. Therefore, I suggest it for publication after minor revision to clarify few parts of the text and increase the potential of the provided information, as follows:

L27. Please add g kg-1 dry residue. Do the same throughout the manuscript.

L70. It would be better to mention that these are conflicting results indicating the effects of other co-factors (e.g. residue properties, native microbial rather than claim that there is no any effect by type of residue placement.

L75-77. “the application of residues in coarser chopped conditions would promote soil aeration to help build healthy soil biota and conserve more water through a consistent breakdown.”. How we can be sure that is the case. Please add references to support it.

L97. to ensure… that. Please modify the sentence, it is too long and the meaning gets confusing.

L101. The study relies on estimates in decomposition rates. Therefore I would suggest to modify from “decomposition” to decomposition rate.

L. 247-254. Figure captions should be at the end of the manuscript followed by figure. Do the same for tables.

Please reform the tables by reducing the lines. See other examples from the studies published in the journal.

Figure 3. Please add st. error or st. dev to the figure to highlight variation of the data.

L322-324. In reasons behind the increased initial decomposition rates I would add the presence of labile organic material (compounds) that fist undergo decomposition by microbial activity. It is mentioned few lines below in the text.

L335-337. This isn’t clear. Please explain further and use references.

L368-369. Please discuss more and provide explanation before you suggest that “Thus, the fine-sized grinding of residue provided no advantage over the chopping of residues.”.

L374-379. Please do the same as above. Strengthen discussion regarding the lack of difference between soil types. Apparently several factors may have been involved. You might discuss the similarity regarding the soil texture due to relatively low clay content or similar low organic matter content (0.9-1.5%)(that may drive aeration-moisture etc.) as well as you could recall similar incubation conditions. On the other hand the existing differences between soils (biological properties are unknown) somehow may mask potential differences in the response of the added residue as you have mentioned. Please elaborate more and use references.

L390-392. Besides the general suggestion for future microbial community analyses you might be more specific by proposing estimates in population of heterotrophic (micro)organisms (fungi and different bacterial phylum) and information regarding their community composition and structure across different soil types (possibly with more profound differentiations in soil properties than the soils you used). For example differences in organic matter content and in texture profile of between fine-textured and sandy soils. Furthermore, field studies are always welcome along with microcosm experimentations to provide information under realistic conditions and crop presence, as you have mentioned in conclusion section. From another perspective it would be interesting to see how the different residue management affect coupling between decomposition and nitrification or denitrification processes. Such information as above may be included in the conclusion section.

L397-398. Indeed. Please reform this sentence by using the term “priming effect”-increased decomposition of the native organic matter, add references.

Reviewer #2: The manuscript "Evaluation of residue management practices on barley residue decomposition" assess how quick barley residue decay depending on size, placement and soil type. These factors are critical in order to understand the nutrient release for subsequent crops and at the same time carbon (C) capture/loss. The authors have carried out an experiment under laboratory conditions, measured C mineralization and thereafter applied the first-order decay kinetics model to calculate time of decomposition. The weakness of this manuscript is that the experiment is done in the laboratory and probably at too high temperatures 25-30°C therefore the results presented in this study may be in discrepancy with similar studies in the field. Moreover, the manuscript lucks discussion about the role of C capture in cereal dominated agricultural systems, particularly how to increase carbon sequestration instead of to lose it. The manuscript needs revision before it could be accepted for publishing.

Introduction

Generally. Please provide information about soil organic carbon (SOC) in Idaho soils. What is average?

Did you have any hypothesis before you started the experiment?

Line 79 Do you mean soil temperature?

Materials and methods

In M&M, there is a need for more detailed information.

Line 105 – What do you mean with ‘’ a common soil series’’? Please, describe in short pre-history of soil you use in the experiment. What was grown before in a sampling location?

Line 109 – You sampled from 0-15 cm depths, why not from 0-20 cm? What is common soil tillage depth?

Table 1 – Please provide in the text how do you calculate soil organic matter (SOM)? You can replace SOM with the loss on ignition (LOI), which you measured.

Line 144 – Please explain why do you choose so high temperature and was it soil or air temperature? How temperature between 25-30 °C can be a constant?

Line 147 How deep was soil lower in a jar? May you provide diameter of the jar and height?

Line 152-159. Please provide more detailed information about:1) where and how did you place a Petri dish – up, down? For how long time? How frequently you changed them?

Line 164-165. Did you add water to jars containing only soil?

Results

‘’Characterization of soils’’ belongs to Materials and methods. Moreover, you have already described this in Table 1 and there is no need to repeat it. Please delay lines 211-220.

Line 222 – How 1 kg barley residue can contain 3875g (3kg 875 g) total carbon (TC)? Did you analyzed C- content or you just calculated? Using your data C:N ratio then is 95:1, if TN is 41g/kg. There is something wrong.

Line 276 – ‘’where’’ written two times.

Line 285 – only ‘’observed’’ or measured/calculated?

Line 295-299. Here and elsewhere too long sentences and sometimes difficult to understand what you mean. Please rephrase it. You should explain in text what you mean with CI. Better use ‘’± ‘’. There is also no need to write ‘’goodness of fit’’, just R2=0.997.

Table 4 Do you need all text under the table? You have described calculations before in M&M.

Discussion

In general manuscript miss discussion about advantages and disadvantages of breakdown barley residue in production and environmental perspective. How real is 25-30 °C in field? How many days with such temperature can you expect?

Line 390 Did you have microbial community analyses? You haven’t mentioned it earlier.

Line 397 At least you mention building up organic matter. You should discuss more about challenges in cereal production.

Refernces

Line 444 and 461. References 10 and 16 are the same.

Supporting information

Lines 516 – 526 can be moved to Materials and methods as it is important information to the reader.

**********

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Reviewer #2: No

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7 Apr 2020

Dear Editor and Reviewers,

Thank you for the opportunity to revise our manuscript, “Evaluation of residue management practices on barley residue decomposition.” We would like to thank the reviewers for their positive comments and have made corrections or addressed all of the reviewers’ comments as noted below. We have provided a track-changes version for the editor and Reviewers.

Regards,

Biswanath Dari (Corresponding Authors)

Following changes were made in general in this revised version of the manuscript:

1. The 'Reference' list has been updated with new references as suggested and appropriate. Accordingly, the order of citation in the main text has changed. We did not maintain the track changes in the ‘Reference’ section because there was quite a change and it would have been crowded if we had to do it in track changes.

2. The sub-section ‘Calculation of CO2-C decomposed’ section has been moved from the ‘Supplementary section to the main text under the ‘Calculation’ section.

3. The sub-section ‘Characteristics of Soils’ section under the ‘Results’ has been deleted as per the suggestion by Reviewer.

4. Table 4 has been modified as per the suggestion by the reviewer as well as for better representability (we did not keep the track changes here).

5. The error bars have been added in Figure 4.

Reviewer: 1

General Comments

The study investigates the effect of three different factors, a. residue placement (surface vs. incorporated), b. residue size (chopped vs. ground sieved) and c. soil type (sand and sandy loam) on barley residue decomposition rates by using a specific modeling approach.

In general, introduction section covers sufficient background knowledge regarding the objectives of the study. Moreover materials and methods are clear and easily captured by the reader. In this line, properties of soils and residue used are properly described by the study. Results and discussion are also adequate. However, more detailed discussion could strengthen the results and provide the basis for the future work that is needed.

Overall, the manuscript is interesting and provide possible options regarding the effects of residue management in decomposition rates of residues left from barley cultivations. Therefore, I suggest it for publication after minor revision to clarify few parts of the text and increase the potential of the provided information, as follows:

Response: We would like to thank the reviewers 1 for the positive comments as well as detailed specific comments that we believe have improved the manuscript. The authors have listed the major requested points from Reviewer 1 below and our responses (Red). All minor comments were also addressed as noted by the track changes document.

Specific Comments

L27. Please add g kg-1 dry residue. Do the same throughout the manuscript.

Response: Thank you for pointing this out. Yes, we had calculated everything based on dry residue. Now the word ‘dry residue’ has been added after ‘g kg-1’ throughout the manuscript.

L70. It would be better to mention that these are conflicting results indicating the effects of other co-factors (e.g. residue properties, native microbial) rather than claim that there is no any effect by type of residue placement.

Response: Thanks for the suggestion. The sentences has been modifies as (Page 4; Line 74-77): ‘The method of residue placement (surface vs. incorporated) and consequently, the contact between the soil and the residue have had conflicting results indicating the effects of other confounding factors (i.e. residue properties, native microbial communities) on the residue breakdown rate in previous research.’

L75-77. “The application of residues in coarser chopped conditions would promote soil aeration to help build healthy soil biota and conserve more water through a consistent breakdown.”. How we can be sure that is the case. Please add references to support it.

Response: We have added the reference (Sims and Frederick, 1970) to support our statement. The statement reads as follows now (Page 4; Line 83-86): ‘In contrast, the application of residues in coarser chopped conditions would result in reduced surface contact between the soil and barley straw which has been claim to slow down the rate of residue breakdown by microbial decomposers [17]’.

L97. To ensure… that. Please modify the sentence, it is too long and the meaning gets confusing.

Response: We have re-written the statement using two sentences as follows (Page 6; Line 110-114): ‘Additionally, the effect of post-harvest management practices of barley residues in irrigated production systems effect on C-mineralization needs to be quantified and understood to ensure sustainable barley production. Achieving sustainable barley production through understanding the dynamics of residue carbon breakdown and residue CO2-C loss nutrient availability (for example, C, N etc.) for subsequent crop production is the focus of this study’.

L101. The study relies on estimates in decomposition rates. Therefore, I would suggest to modify from “decomposition” to ‘decomposition rate’.

Response: We have changed the word ‘decomposition’ to ‘decomposition rate’.

L. 247-254. Figure captions should be at the end of the manuscript followed by figure. Do the same for tables.

Response: Thank you for the suggestion. However, we have followed the most recent pattern of organizing the manuscript as per our understanding of the PLOS One journal editorial guidelines. We believe the guidelines state that figure caption and tables (with table caption) should be placed after their first mention in the main text, and the figures should be submitted as a separate file. If we have misunderstood the PLOS One journal guidelines we are more than happy to correct this issue.

Figure 3. Please add st. error or st. dev to the figure to highlight variation of the data.

Response: We already have standard errors (SE) for Figures 1 and 2. The value of SE is smaller so it is hard to see in the figures. Now, we have added SE for Figure 3 as well. .

L322-324. In reasons behind the increased initial decomposition rates, I would add the presence of labile organic material (compounds) that first undergo decomposition by microbial activity. It is mentioned few lines below in the text.

Response: We have modified the sentence and the sentence reads as follows (Page 21; Line 373-375): ‘This is because the maximum decomposition rate occurs in this period due to soil nutrient dynamics, moisture availability (and soil aeration status), and labile organic material that first undergo decomposition by triggering microbial activity’.

L335-337. This isn’t clear. Please explain further and use references.

Response: The sentences has been re-written as follows (Page 21-22; Line 389-394): ‘Enhancing the physical contact between crop residues and soil minerals may have further promoted the residue-mineral interactions, resulting in lower cumulative decomposition in the incorporated treatment than the surface treatment [5,11,37]’.

L368-369. Please discuss more and provide explanation before you suggest that “Thus, the fine-sized grinding of residue provided no advantage over the chopping of residues.”

Response: We have modified this section and now reads as follows (Page 23; Line 430-437): ‘Differences were observed in the initial decomposition rate through day-5 which afterwards disappeared. In addition, the amount of C that were not mineralized in the initial period decomposition (~ 5 days) remained quite stabilized in the soil for longer duration after initial flush of CO2-C release. Thus, the fine-sized grinding of residue provided no advantage over the chopping of residues in terms N release (and subsequent crop uptake) from decomposing residue for sustainable agricultural production [11]’.

L374-379. Please do the same as above. Strengthen discussion regarding the lack of difference between soil types. Apparently several factors may have been involved. You might discuss the similarity regarding the soil texture due to relatively low clay content or similar low organic matter content (0.9-1.5%) (That may drive aeration-moisture etc.) as well as you could recall similar incubation conditions. On the other hand the existing differences between soils (biological properties are unknown) somehow may mask potential differences in the response of the added residue as you have mentioned. Please elaborate more and use references.

Response: Thanks for the comment. It helps to improve this section of the discussion. Now, we have modified the paragraphs as follows (Page 24; Line 442-454): ‘Our study did not show any significant differences in the cumulative CO2-C release by the two soils (Table 3). These two soils possess similar soil texture, however, these soils were selected as they are representative of the relatively similar soils found in the study region (south-eastern Idaho) (Table 1). Due to their similar soil texture, the CO2-C mineralized from both the soils irrespective of the type of residue added were similar. This might be due to the fact that both soils have lower clay or organic matter content (0.9-1.5%) which has influenced the aeration and moisture status of the soil-residue mixtures [19]. Further, the presence of clay mineral (organo-clay mineral association) controls the magnitude of residue-mineral interactions which was not significantly different between these two soil types as both are relatively low in clay content. Additionally, other associated soil biological factors such as SOM did not vary widely which could have possibly made a difference in microbial populations and nutrient dynamics in these two soil types, and thus, differences in CO2-C release from the soils’.

L390-392. Besides the general suggestion for future microbial community analyses you might be more specific by proposing estimates in population of heterotrophic (micro)organisms (fungi and different bacterial phylum) and information regarding their community composition and structure across different soil types (possibly with more profound differentiations in soil properties than the soils you used). For example, differences in organic matter content and in texture profile of between fine-textured and sandy soils. Furthermore, field studies are always welcome along with microcosm experimentations to provide information under realistic conditions and crop presence, as you have mentioned in conclusion section. From another perspective it would be interesting to see how the different residue management affect coupling between decomposition and nitrification or denitrification processes. Such information as above may be included in the conclusion section.

Response: There are great suggestions and really helpful. As per the suggestions, we have modified the sentence as follows (Page 25; Line 465-470): ‘Further microbial community analyses which estimates the population of heterotrophic microorganisms (e.g. fungi, different bacterial phylum, etc.) and information regarding their community composition and structure (e.g. bacterial:fungal ratio determination using Phospholipid-derived fatty acid; PLFAs [40]) across different soil types might explain the variability in k values for various treatment combinations’.

We have now modified/added the following statement in the conclusion section as follows (Page 26; Line 494-502): ‘However, barley residue decomposition study under field conditions along with microcosm experimentation in laboratory, and measured at varying times during the crop growing season is warranted to achieve a definite conclusion. Besides, the inclusion of residues in soils with varying SOM content and/or texture profile (e.g. fine- vs. coarse-textured) would be helpful to achieve a definite conclusion. As recommendations are developed, they should consider the advantages and disadvantages of faster or slower residue breakdown under field condition, which is location- and soil- specific. Thus, it would be interesting to see if various residue management practices have coupling effects on soil C (soil C formation and loss) and N cycle (nitrification and denitrification) processes’.

L397-398. Indeed. Please reform this sentence by using the term “priming effect”-increased decomposition of the native organic matter, add references.

Response: Thanks for this great comments. We have modified the sentence and added a sentence. The statements reads as follows (Page 24-25; Line 475-480): ‘However, faster residue mineralization might have a priming effects by increasing the decomposition of native organic matter, thus, reducinge the soil's organic matter buildup [41]. Results indicated that we can achieve a faster C-mineralization rate of for surface-chopped residues mixed with soils under controlled laboratory conditions over a period of 50-d. However, a future study would be warrant to capture the priming effects and it would require labelling of either soil or the residue’.

Reviewer: 2

General Comments

The manuscript "Evaluation of residue management practices on barley residue decomposition" assess how quick barley residue decay depending on size, placement and soil type. These factors are critical in order to understand the nutrient release for subsequent crops and at the same time carbon (C) capture/loss. The authors have carried out an experiment under laboratory conditions, measured C mineralization and thereafter applied the first-order decay kinetics model to calculate time of decomposition. The weakness of this manuscript is that the experiment is done in the laboratory and probably at too high temperatures 25-30°C, therefore, the results presented in this study may be in discrepancy with similar studies in the field. Moreover, the manuscript lacks discussion about the role of C capture in cereal dominated agricultural systems, particularly how to increase carbon sequestration instead of to lose it. The manuscript needs revision before it could be accepted for publishing.

Response: We would like to thank the reviewers 2 for the positive comments, criticism, as well as detailed specific comments that we believe, have improved the manuscript. The authors have listed the major requested points from Reviewer 2 below and our responses (Red). All minor comments were also addressed as noted by the track changes document.

We have re-written the following sentence to explain this further in the sub-section ‘Laboratory incubation experimental approach’ under the section ‘M&M’ as follows (Page 8-9; Line 165-168): ‘A 50-d laboratory incubation experiment was conducted at a constant temperature (25°C) and moisture (60% water-filled pore space). Although we did not mimic the field condition, the experimental set-up we used allowed us to compare our findings with other microcosm studies focused on residue decomposition [5,18]’.

Temperature was a constant factor in the experiment and thus, was not manipulated for the experiment. As a laboratory experiment the temperature was set at an optimal range of both moisture and temperature to determine differences under these conditions similar to previous work (Gilmour et al., 1998, Al Kaisi et al., 2016, 2017, Rogers et al. 2017, etc.). To elaborate, the study was conducted at an air temperature of 25°C under laboratory conditions which is within the range of summer field conditions (June to August) in the study region and in many temperate regions. We aimed to conduct this study with air temperature of 25°C which optimizes the microbial activity and captures the maximum decomposition i.e. C-mineralization (Gilmour et al. 1998). Thus, this temperature condition can be related to how different residue type impact decay constant i.e. k values.

We have corrected the temperature as a 25°C temperature was maintained in the incubator throughout the experiment. We have corrected this issue in the main text.

We have tried our best to explain by reasoning the difference in C-mineralization (i.e. residue decomposition) under various soil-residue mixtures in a barley (as a cereal crop) production system. The main purpose of this study was to show the CO2-C release and how it differs as per various barley residue types. We did not intend to measure soil carbon sequestration which was beyond the scope of this study. Our study warrant future studies which could involves evaluating soil carbon capture/carbon storage due different management practices (e.g. tillage). Carbon being captured in the soil is affected by management practices and may influence carbon sequestration. However, post-harvest residues from these cereal crops can limit available N in the soil due to high C: N ratios. To decompose residues with high C: N ratios, growers may attempt different types of tillage. Tillage can affect the amount of residue cover left on the field and is variable depending on management preferences. Each type of tillage management will affect the status of residue cover and the amount of residue-soil contact. For example, when using a moldboard plow for corn/small grain residue, there is only 0 to10 percent of residue cover left in the field (USDA, 2018). In contrast, using a disk (offset, primary >9” spacing) or a chisel will result in 40 to80 percent of residue cover leftover on the soil surface (USDA, 2018). Management of residue cover is ultimately determined by what is available mechanically by the grower.”

Reference: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/technical/nra/rca/?cid=nrcs144p2_027241

Introduction

Generally. Please provide information about soil organic carbon (SOC) in Idaho soils. What is average?

Response: The average soil organic carbon content of Idaho soils from previous studies in the predominant agriculture production areas is 2 (±0.1) g kg-1 (Rogers et al. 2019). The detailed information can be found in our other publication at Rogers et al., 2019. We have added the following sentence in the ‘Site Description’ sub-section under the section ‘M&M’ as follows (Page 6; Line 125-127): ‘The average soil organic carbon content measured by loss on ignition (LOI) in the study region was 2 g kg-1 [23]’.

Comment: Did you have any hypothesis before you started the experiment?

Response: Thanks for your suggestion. Yes we had hypotheses before we started, however, we have chosen to simply state the research questions and objectives instead of hypothesis. We would be more than happy to add the hypotheses, if further needed.

Line 79. Do you mean soil temperature?

Response: We have modified the sentence to capture the status of soil temperature as suggested in the reference. We have re-written the sentence as follows (Page 5; Line 87-90): ‘Thus, smaller sized residues may favor C-mineralization and residue breakdown in barley by either increased microbial activities particularly by increasing soil temperature [15] or by filling the soil pore-space in higher magnitude which facilitate decomposition [5]’.

Materials and methods

In M&M: There is a need for more detailed information.

Line 105 – What do you mean with ‘’ a common soil series’’? Please, describe in short pre-history of soil you use in the experiment. What was grown before in a sampling location?

Response: Thanks for the comment. However, we are not clear on this comment as we have already mentioned about the soil series with references such as (Page 6; Line 121-123): ‘Two soil samples (with four field replicates) were collected from a common soil series (Declo Loam; Coarse-loamy, mixed, superactive, mesic Xeric Haplocalcids) in southeastern Idaho [21,22] (Table 1)’. We are not certain on what other info can be added here. Additionally, we have added the reference to the USDA-NRCS soil classification system. I hope it helps. We would be happy to provide more info if applicable.

We have added one sentence on the type of crops grown in the soils before sampling as (Page 6; Line 124-125): ‘Before soil sample collection, the lands were cultivated with barley (Hordeum vulgare, L.)’.

Line 109 – You sampled from 0-15 cm depths, why not from 0-20 cm? What is common soil tillage depth?

Response: Thanks for this comment. We have added the following sentence along with a reference to explain as follows (Page 6; Line 129-130): ‘Tillage operations can be highly varied in Idaho and other western States. The depth of 0-15 cm is an approximation for shallow tillage operations that are conducted using disc plows [5]’.

Table 1: Please provide in the text how do you calculate soil organic matter (SOM)? You can replace SOM with the loss on ignition (LOI), which you measured.

Response: We have kept the SOM values as measured by LOI methods throughout the manuscript to maintain consistency and avoid any confusion.

Line 144. Please explain why do you choose so high temperature and was it soil or air temperature? How temperature between 25°C can be a constant?

Response: Please see our responses to the general comment by Reviewer 2 as well. We acknowledge the fact that the study was conducted at a constant air temp of 25°C under laboratory conditions (at an incubator). The goal of the study was to optimizes the microbial activity and captures the maximum decomposition i.e. C-mineralization in the study region (Gilmour et al. 1998). Thus, this temperature condition can be related to how different residue type impact decay constant i.e. k values. We have re-written the following sentence to explain this further in the sub-section ‘Laboratory incubation experimental approach’ under the section ‘M&M’ as follows (Page 8-9; Line 165-168): ‘A 50-d laboratory incubation experiment was conducted at a constant temperature (25°C) and moisture (60% water-filled pore space). Although we did not mimic the field condition, the experimental set-up we used allowed us to compare our findings with other microcosm studies focused on residue decomposition [5,18]’.

Line 147. How deep was soil layer in a jar? May you provide diameter of the jar and height?

Response: The information are provided as follows (Page 9; Line 174-179): ‘Barley residue (4.1 g) and soil (100 g) were placed in a Mason jar. A 500 mL capacity wide-mouth mason jars with an 86 mm dome lid modified to support a 60-mm petri dish (32) were used in this study. The inner diameter width 76 cm, outer diameter width of 82 cm, and height of 113cm. (The inner diameter width 76 cm, outer diameter width of 82 cm, and height of 113cm)’.

The depth of soil-residue mixture was approximately 35-45 cm from the bottom of the jar.

Line 152-159. Please provide more detailed information about: 1) where and how did you place a Petri dish – up, down? For how long time? How frequently you changed them?

Response: Thanks for your comments. We have re-written and added one sentence in main text as follows (Page 9; Line 182): ‘The study was conducted based on incubation methods described by [5]. A statement describing the frequency of change in Petri-dishes are also mentioned in the text already as follows (Page 9; Line 189-190): ‘A new petri dish with 1 M NaOH was used at each time interval after taking the previous petri-dish from individual mason jars’. We would be more than happy to provide further details if needed. The mason jars were constructed such a way so that it allows the Petri dishes to be suspended above the soil.

Line 164-165. Did you add water to jars containing only soil?

Response: Yes, we have added water to the jars containing soil-residue mixture or soils only to retain soil moisture at optimal levels. This includes both soil with residue added and soil without residue added which were both needed to make accurate calculations of specific losses from residue as opposed to those from the actual soil.

Results

“Characterization of soils”: Belongs to Materials and methods. Moreover, you have already described this in Table 1 and there is no need to repeat it. Please delete lines 211-220.

Response: We have deleted the entire sub-section ‘Characteristics of Soils’ under the ‘Results’ section.

Line 222. How 1 kg barley residue can contain 3875g (3kg 875 g) total carbon (TC)? Did you analyzed C- content or you just calculated? Using your data C:N ratio then is 95:1, if TN is 41g/kg. There is something wrong.

Response: We apologize for our typo. Now, we have corrected the values in Table 2 (Page 14) and the main texts.

Line 276. ‘Where’ written two times.

Response: One extra ‘where’ has been deleted.

Line 285. Only ‘observed’ or measured/calculated?

Response: It would be measured. It has been corrected now.

Line 295-299. Here and elsewhere too long sentences and sometimes difficult to understand what you mean. Please rephrase it. You should explain in text what you mean with CI. Better use ‘’± ‘’. There is also no need to write ‘’goodness of fit’’, just R2=0.997.

Response: Thanks for the positive note here. We have modified CI into ‘±’ throughout the manuscript including modified Table 4 (Page 19-20) and in the section ‘First-order decay kinetics and half-life of barley residues’ under the section ‘Results’ (Page 17; Line 313-333). We have used only ‘R2’ instead of ‘goodness of fit’ throughout the text and under the section ‘First-order decay kinetics and half-life of barley residues’ (Page 17). We have either re-phrased or split the sentences under the section ‘First-order decay kinetics and half-life of barley residues' (Page 17) for better readability. Please see the modified Table 4.

Table 4. Do you need all text under the table? You have described calculations before in M&M.

Response: Thanks for the note. We have deleted most of the footnote from under Table 3 (Page 16-17) and Table 4 (Page 19-20) except that are needed which were already explained in the section ‘Calculation’ under the section ‘M&M’.

Discussion

Comments: In general manuscript miss discussion about advantages and disadvantages of breakdown barley residue in production and environmental perspective. How real is 25-30°C in field? How many days with such temperature can you expect?

Response: Please see our responses for the general comment and comment given on Line 144. We hope it will clarify. We would be happy to provide further details if needed.

Line 390. Did you have microbial community analyses? You haven’t mentioned it earlier.

Response: We did not perform any microbial community analyses for this study. However, as we indicated our responses to the comment for Line 390-392 by Reviewer 1 on this same comment we believe it could be of benefit in future studies. We hope this will help.

Line 397. At least you mention building up organic matter. You should discuss more about challenges in cereal production.

Response: The focus of this study was on the decomposition of the actual residue and focus on specific organic matter buildup was beyond the results measured in the study. Additionally, the timeframe of the experiment, and the long duration to influence SOM result in data beyond the scope of the study. However, we agree that soil organic matter would be influenced by the results of the decomposition of the cereal straws but are hesitant to go further than we have with the data that was produced in this laboratory study. We are unclear on what specific challenges the reviewer is interested in and thus, are uncertain of what they are requesting we add without more details.

References

Line 444 and 461. References 10 and 16 are the same.

Response: Thanks for catching the error. The duplicate reference has been deleted from the ‘Reference’ list and the citation order in the main text and ‘Reference’ list has been updated accordingly.

Supporting information

Lines 516 – 526. Can be moved to Materials and methods as it is important information to the reader.

Response: This section has been moved to the main text in the sub-section ‘Calculation’ under the section ‘M&M’ after the subsection ‘First-order decay constant calculation’.

Submitted filename: Response To Reviewers.docx

Click here for additional data file.

24 Apr 2020

Evaluation of residue management practices on barley residue decomposition

PONE-D-19-33598R1

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Reviewer #1: Thank you for your response to my comments.

L75. Please remove “in previous research”.

L83. “soil temperature” isn’t possible discrimination factor between differentially sized residues. In my opinion it would be avoided.

Styles of the Tables used could be changed.

Reviewer #2: The Authors have taken into consideration the comments and have improved the manuscript. This manuscript is now acceptable for publication after minor revision. Pge 9, lines 177-179 the same text is written twice.

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28 Apr 2020

PONE-D-19-33598R1

Evaluation of residue management practices on barley residue decomposition

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