Influence of Differ P Enrichment Frequency on Plant Growth and Plant C:N:P in a P-Limited Subtropical Lake Wetland, China.

Phosphorus (P) enrichment as a result of anthropogenic activities can potentially alter plant C:N:P stoichiometry. However, the influence of different P enrichment frequencies on plant C:N:P stoichiometry in P-limited ecosystems is still unclear. In this study, we conducted a P-addition experiment to elucidate the effect of various P enrichment frequencies on the plant C:N:P stoichiometry of Carex brevicuspis in a freshwater wetland at Dongting Lake, China. We used four P enrichment frequencies (treatment A: no P addition; treatment B: three 0.1 g kg-1 additions at 10-day intervals; treatment C: two 0.15 g kg-1 additions at 15-day intervals; and treatment D: one 0.3 g kg-1 addition during the experimental period) in a factorial design with an experimental duration of 30 days. Biomass accumulation was lowest in the treatment A and highest in the treatment C, and increased with decreasing P addition frequency. The shoot:root ratio did not differ significantly between the four treatments. Both foliar and root C concentrations were not significantly different between the treatments. Foliar N concentration was significantly lower in the treatment D than in the other three treatments, while root N concentration did not differ significantly between the treatments. Both foliar and root P concentrations, and foliar C:N were much higher in the treatment B than in the treatment A. However, root C:N did not differ significantly between treatments. Both foliar and root C:P and N:P of C. brevicuspis were lower in the treatment B than in the treatment A. These results indicated that different frequencies of P addition significantly influenced plant growth. Moreover, P enrichment, rather than frequency, significantly influenced plant C:N:P stoichiometry. Our results improve our understanding of the influence of different P enrichment frequencies on plant C:N:P stoichiometry and nutrient cycling in freshwater wetlands.

Introduction

n class="Chemical">Phosphorus (P) plays aclass="Chemical">n importaclass="Chemical">nt role iclass="Chemical">n determiclass="Chemical">niclass="Chemical">ng placlass="Chemical">nt growth aclass="Chemical">nd commuclass="Chemical">nity structure iclass="Chemical">n both terrestrial aclass="Chemical">nd aquatic ecosystems (Elser et al., 2007; He aclass="Chemical">nd Dijkstra, 2015; Mao et al., 2016). Receclass="Chemical">ntly, aclass="Chemical">nthropogeclass="Chemical">nic discharges of P have doubled the class="Chemical">natural P loadiclass="Chemical">ng due to the iclass="Chemical">nteclass="Chemical">nsity of activities such as aquaculture aclass="Chemical">nd agriculture (Mao et al., 2015). These large quaclass="Chemical">ntities of P are discharged iclass="Chemical">nto wetlaclass="Chemical">nd ecosystems, leadiclass="Chemical">ng to a series of ecological aclass="Chemical">nd eclass="Chemical">nviroclass="Chemical">nmeclass="Chemical">ntal problems, such as eutrophicatioclass="Chemical">n aclass="Chemical">nd biodiversity reductioclass="Chemical">n (Rashid aclass="Chemical">nd Romshoo, 2013; Bu et al., 2016).

Relative abundances of class="Chemical">carbon (C), class="Chemical">n class="Chemical">nitrogen (N), and P, and C:N:P stoichiometry in plants are powerful indicators of ecological processes, e.g., community organization, nutrient limitation, food webs, and decomposition (Elser et al., 2000; Güsewell et al., 2003; Xia et al., 2014). Therefore, studies on plant ecological stoichiometry enhance our understanding of the growth and nutrient-use strategies of plants, and their responses to various environmental stresses. For example, plant C:N:P stoichiometry is significantly affected by external nutrient availability (Elser et al., 2007; Mao et al., 2016). To date, the influence of increased P loading on plant C:N:P stoichiometry has been studied in various wetland ecosystems (Rejmánková et al., 2008; Mao et al., 2016). Most of these studies confirmed that increasing P loading enhanced plant P concentration and decreased C:P, while results regarding the influence of increasing P loading on plant N concentration, C:N, and N:P were inconsistent across studies (Mao et al., 2016). In P-limited ecosystems, P enrichment would promote plant growth and reduce plant N concentrations and N:P ratios, mainly due to the dilution effect (Feller et al., 2007; Mao et al., 2016). However, in N-limited ecosystems, the response of plant N concentration, C:N ratio and N:P ratio might be determined by the plant’s nutrient use strategies (Yuan and Chen, 2015; Mao et al., 2016). Therefore, more studies are needed to investigate the general influence of P enrichment on plant C:N:P stoichiometry.

Nutrient concentrations can increase at different rates over different spatiotemporal scales, which influences plant growth performance (Xie et al., 2004; Zhang et al., 2018). For instance, biomass allocation patterns and P allocation ratios in class="Species">Eichhornia crassipes differed sigclass="Chemical">nificaclass="Chemical">ntly with differeclass="Chemical">nt modes of class="Chemical">nutrieclass="Chemical">nt iclass="Chemical">ncrease (Xie et al., 2004). Differeclass="Chemical">nt class="Chemical">nutrieclass="Chemical">nt eclass="Chemical">nrichmeclass="Chemical">nt rates would alter soil class="Chemical">n class="Chemical">nitrogen-phosphorus imbalances, affecting the structure, function, and diversity of ecosystems and organisms (Peñuelas et al., 2013). Sun et al. (2018) also confirmed that different N and P input ratios significantly changed the levels of N, P, and other elements in plants, and that the influence differed among different plant organs. While many studies have investigated the influence of P enrichment on plant C:N:P stoichiometry in different types of wetlands (Newman et al., 2004; Feller et al., 2007; Mao et al., 2016), the effects of different P enrichment frequencies on plant stoichiometry remain unclear.

This study investigated the effects of different P enrichment frequencies on wetland plant stoichiometry at Dongting Lake wetland, China. This lake is the second largest freshclass="Chemical">water lake iclass="Chemical">n Chiclass="Chemical">na aclass="Chemical">nd has the largest class="Chemical">n class="Chemical">water exchange capacity with the Yangtze River (Xie and Chen, 2008). Our previous studies confirmed that plants were limited by P in this lake (Li et al., 2017, 2018), but it has received increasing inputs of P mainly due to the use of agricultural fertilizer in the local area. The quantity of P input into the lake is about 4.1 × 104 t annually (He et al., 2009). Here, we report the changes in plant C:N:P stoichiometry and growth performance of Carex brevicuspis after four different P enrichment frequency treatments. Based on the above statement, we hypothesized that (1) P enrichment would promote the growth of C. brevicuspis and the influence differ with different P enrichment treatments; (2) P enrichment would increase plant P concentration and C:N ratio, but reduce plant N concentration, C:P and N:P ratios. Moreover, these influences would differ with different P enrichment frequencies.

Materials and Methods

Study Site

Dongting Lake (28°30′–30°20′ N, 111°40′–113°10′ E) is located on the south bank of the mid-section of the Yangtze River (Xie and Chen, 2008). The lake receives inflow from four rivers (Xiang, Zi, Yuan, and Li) in Hunan Province and four channels (Songzikou, Taipingkou, Ouchikou, and Tiaoxiankou) connect it to the Yangtze River (Figure 1). The wetlands are characterized by large seasonal fluctuations in n class="Chemical">water level aclass="Chemical">nd are usually completely flooded from May to October aclass="Chemical">nd susceptible to drought from November to April. The meaclass="Chemical">n aclass="Chemical">nclass="Chemical">nual temperature is 16.8°C, with hot summers (Juclass="Chemical">ne to August, 27.3°C) aclass="Chemical">nd cold wiclass="Chemical">nters (December to February, 5.8°C). Aclass="Chemical">nclass="Chemical">nual precipitatioclass="Chemical">n is 1382 mm, with more thaclass="Chemical">n 60% falliclass="Chemical">ng from April to August (Declass="Chemical">ng et al., 2015).

FIGURE 1

Dongting Lake, showing the location of the study site. The shaded areas represent the wetlands.

Study Species

Carex brevicuspis is widely distributed in Taiwan and eastern mainland China (Dai et al., 2010). At Dongting Lake, this species can form mono-dominant communities or co-exist with other species e.g., class="Species">Miscanthus sacchariflorus aclass="Chemical">nd class="Chemical">n class="Species">Polygonum hydropiper. C. brevicuspis usually has two growing phases which are related to changes in the water rhythm in Dongting Lake, usually flowering and fruiting in April or May before the flooding, and remaining completely submerged during the flooding season. After flooding, the shoots emerge immediately (November) and grow until January. In January, the above-ground plant parts wither. Subsequently, new ramets emerge and grow rapidly in February and/or March (Deng et al., 2015). In the Dongting Lake, the C. brevicuspis community plays an important role in biodiversity maintenance owing to its multiple ecological functions, for example as a food resource for migratory birds and a spawning ground for migratory fish.

Plants were collected in May 2017 from Junshan county (29°22′ N, 112°59′ E), East Dongting Lake wetland. Small blocks (25 × 25 cm) of class="Species">C. brevicuspis vegetatioclass="Chemical">n were cut aclass="Chemical">nd traclass="Chemical">nsported to aclass="Chemical">n experimeclass="Chemical">ntal field at the Doclass="Chemical">ngticlass="Chemical">ng Lake Statioclass="Chemical">n for Wetlaclass="Chemical">nd Ecosystem Research, Chiclass="Chemical">nese Academy of Scieclass="Chemical">nces. The blocks were placed iclass="Chemical">n plastic buckets (87 × 65 × 62 cm), which coclass="Chemical">ntaiclass="Chemical">ned 20 cm soil (7.07 mg g−1 class="Chemical">n class="Disease">organic matter, 0.87 mg g−1 total N, and 0.73 mg g−1 total P; Table 1) to germinate new ramets. Soil was also collected from the area (0–20 cm depth) in which the C. brevicuspis plants were collected. The plants were watered once weekly with tap water (0.511 μg L−1 NH4+−N, 1.760 μg L−1 NO3−N, 0.527 μg L−1 PO43+−P, pH = 7.2).

Table 1

Soil characteristics (mean ± SE) after different P addition frequency treatments (treatments A–D represent: no P addition treatment; three – time P addition treatment; two-time P addition treatment; and one-time P addition treatment, respectively).

Treatments	Total nitrogen content (mg g−1)	Total phosphorus content (mg g−1)	Organic carbon content (mg g−1)	C:N	C:P	N:P
A	0.87 ± 0.02a	0.73 ± 0.01b	7.07 ± 0.14	8.20 ± 0.14b	9.69 ± 0.15	1.18 ± 0.02a
B	0.82 ± 0.02ab	0.77 ± 0.01ab	7.23 ± 0.15	8.87 ± 0.14ab	9.42 ± 0.17	1.07 ± 0.03ab
C	0.79 ± 0.05ab	0.79 ± 0.02a	7.14 ± 0.25	9.25 ± 0.42a	9.11 ± 0.34	1.00 ± 0.06bc
D	0.72 ± 0.04b	0.77 ± 0.02ab	6.87 ± 0.24	9.64 ± 0.25a	8.92 ± 0.33	0.94 ± 0.05c

Experimental Design

Four P enrichment treatments were applied: treatment A – no P addition; treatment B – three 0.1 g kg−1 P additions at 10-day intervals; treatment C – two 0.15 g kg−1 P additions at 15-day intervals; and treatment D – one 0.3 g kg−1 P addition during the experimental period.

On July 11, 2017, 840 seedlings of similar size (3–4 leaves, 20 ± 3 cm in height) were transplanted into 56 pots (30 cm height, 23 cm diameter, and 15 seedlings per pot), which were filled with 7 kg soil (same soil as for seedling cultivation). All pots were placed into 1 of 7 cement ponds (100 × 100 × 100 cm, two pots per treatment per pond) in a random block design. Experimental treatments began on July 18, 2017. class="Chemical">Phosphorus was added as class="Chemical">n class="Chemical">NaH2PO4. Firstly, the required mass of NaH2PO4 was dissolved in 150 ml tap water and then sprayed uniformly into the pot. For each P addition treatment, the pots that did not receive NaH2PO4 were leached using 150 ml tap water. Each pot was supplied 500 mL tap water once weekly and exposed to natural sunlight.

Harvest

The plants were harvested 30 days after the first P treatment, before they flowered. Plant roots were carefully dug out by hand and rinsed using n class="Chemical">tap water to remove sedimeclass="Chemical">nt. Theclass="Chemical">n, placlass="Chemical">nt parts were separated iclass="Chemical">nto leaves aclass="Chemical">nd roots (root aclass="Chemical">nd rhizome), due to the rhizome is difficult to separated. All parts were oveclass="Chemical">n dried at 80°C for 48 h aclass="Chemical">nd weighed. Biomass accumulatioclass="Chemical">n was calculated as the collective mass of all tissues. Shoot:root ratio was deficlass="Chemical">ned as the ratio of leaf mass to root mass. After placlass="Chemical">nt harvest, we collected soil samples at each pot for soil aclass="Chemical">nalysis.

Laboratory Analysis

Dry foliar and root samples were ground for further analysis. Total N and C concentrations were measured using an elemental analyzer (Vario MAX CN, Elementar, Germany), and total P concentration was measured using the class="Chemical">molybdenum blue colorimetric method after digesticlass="Chemical">ng the samples iclass="Chemical">n a solutioclass="Chemical">n of class="Chemical">n class="Chemical">H2SO4 and H2O2 (Zhang et al., 2015).

Soil samples were air-dried and sieved through a 0.15 mm sieve before analysis. Soil class="Chemical">organic C coclass="Chemical">nteclass="Chemical">nt was measured usiclass="Chemical">ng wet oxidatioclass="Chemical">n of class="Chemical">n class="Disease">organic matter with a solution of KCr2O7 and H2SO4, followed by back-titration using FeSO4. Soil N concentration was measured using the Kjeldahl method and soil P concentration was measured using acid digestion with a solution of H2SO4 and HClO4 (Zhang et al., 2015).

Data Analysis

One-way analysis of variance (ANOVA) was performed in conjunction with Duncan’s test to determine the effect of P addition frequency on biomass accumulation, shoot:root ratio, and plant stoichiometry of class="Species">C. brevicuspis, as well as soil characteristics (total N, total P, class="Chemical">n class="Chemical">organic carbon content, C:N, C:P, and N : P). Tukey’s post hoc tests were used for multiple comparisons. Data were log10-transformed where necessary to reduce the heterogeneity of variance. Liljefor’s test and Levene’s test were used to test the normality and homogeneity of data, respectively. All statistical analyses were conducted using SPSS ver. 15.0 (SPSS Inc., Chicago, IL, United States).

Results

Biomass Accumulation and Shoot:Root Ratio

The P enrichment frequency had a significant influence on the biomass accumulation of n class="Species">C. brevicuspis (F = 9.562; df = 6; aclass="Chemical">nd P < 0.001; Figure 2A), which was lowest iclass="Chemical">n the treatmeclass="Chemical">nt A aclass="Chemical">nd highest iclass="Chemical">n the treatmeclass="Chemical">nt C. Moreover, biomass accumulatioclass="Chemical">n iclass="Chemical">ncreased with decreasiclass="Chemical">ng P eclass="Chemical">nrichmeclass="Chemical">nt frequeclass="Chemical">ncy. Iclass="Chemical">n coclass="Chemical">ntrast, shoot:root ratio did class="Chemical">not differ sigclass="Chemical">nificaclass="Chemical">ntly betweeclass="Chemical">n the four frequeclass="Chemical">ncy treatmeclass="Chemical">nts (F = 0.103; df = 6; aclass="Chemical">nd P > 0.05; Figure 2B).

FIGURE 2

Biomass accumulation (A) and allocation (B) of Carex brevicuspis (means ± standard errors, n = 7) under different P addition frequencies (treatments A–D represent: no P addition treatment; three - time P addition treatment; two-time P addition treatment; and one-time P addition treatment, respectively). Different letters indicate significant differences between treatments at the 0.05 significance level.

Foliar Stoichiometry

The frequency of P addition had no significant influence on the foliar total C concentration of n class="Species">C. brevicuspis (F = 2.041; df = 6; aclass="Chemical">nd P > 0.05; Figure 3A), although it was lower iclass="Chemical">n the treatmeclass="Chemical">nt D thaclass="Chemical">n iclass="Chemical">n the other treatmeclass="Chemical">nts. Foliar total N was sigclass="Chemical">nificaclass="Chemical">ntly affected by P additioclass="Chemical">n frequeclass="Chemical">ncy (F = 3.465; df = 6; aclass="Chemical">nd P < 0.05; Figure 3B), which was much lower iclass="Chemical">n the treatmeclass="Chemical">nt D thaclass="Chemical">n iclass="Chemical">n the other three treatmeclass="Chemical">nts. Foliar total P coclass="Chemical">nceclass="Chemical">ntratioclass="Chemical">n was much higher iclass="Chemical">n the three P additioclass="Chemical">n treatmeclass="Chemical">nts thaclass="Chemical">n iclass="Chemical">n the coclass="Chemical">ntrol (F = 30.219; df = 6; aclass="Chemical">nd P < 0.05; Figure 3C), but did class="Chemical">not differ sigclass="Chemical">nificaclass="Chemical">ntly betweeclass="Chemical">n the three P additioclass="Chemical">n treatmeclass="Chemical">nts.

FIGURE 3

Foliar ecological stoichiometry (A–F) of Carex brevicuspis (means ± standard errors, n = 7) under different P addition frequencies (treatments A–D represent: no P addition treatment; three - time P addition treatment; two-time P addition treatment; and one-time P addition treatment, respectively). Different letters indicate significant differences between treatments at the 0.05 significance level.

Foliar C:N showed a similar trend as foliar P concentration, which was much higher in the three addition treatments than in the control (F = 4.009; df = 6; and P < 0.05; Figure 3D), but did not differ significantly among the three P addition treatments. Both foliar C:P (F = 47.787; df = 6; and P < 0.05; Figure 3E) and foliar N:P (F = 61.053; df = 6; and P < 0.05; Figure 3F) showed similar trends, and were much lower in the three P addition treatments than in the control.

Root Stoichiometry

Root total C (F = 2.223; df = 6; and P > 0.05; Figure 4A) and total N (F = 1.299; df = 6; and P > 0.05; Figure 4B) were not significantly different between the treatments. Different P addition treatments significantly influenced the root P content (F = 15.931; df = 6; and P < 0.05; Figure 4C), which was much higher in the three P addition treatments than in the control, and the highest root P content occurred in the treatment C.

FIGURE 4

Root ecological stoichiometry (A–F) of Carex brevicuspis (means ± standard errors, n = 7) under different P addition frequencies (treatments A–D represent: no P addition treatment; three - time P addition treatment; two-time P addition treatment; and one-time P addition treatment, respectively). Different letters indicate significant differences between treatments at the 0.05 significance level.

Root C:N did not differ significantly between the treatments (F = 0.541; df = 6; and P > 0.05; Figure 4D). Both root C:P (F = 31.954; df = 6; and P < 0.05; Figure 4E) and N:P (F = 12.001; df = 6; and P < 0.05; Figure 4F) were significantly lower in the three P addition treatments than in the control, but were not significantly different between the three P addition treatments.

Discussion

Our study confirmed that biomass accumulation in class="Species">C. brevicuspis was much higher iclass="Chemical">n the three P additioclass="Chemical">n treatmeclass="Chemical">nts thaclass="Chemical">n iclass="Chemical">n the coclass="Chemical">ntrol, aclass="Chemical">nd much higher iclass="Chemical">n the two-time P additioclass="Chemical">n treatmeclass="Chemical">nt thaclass="Chemical">n iclass="Chemical">n the three-time P additioclass="Chemical">n treatmeclass="Chemical">nt. These results were coclass="Chemical">nsisteclass="Chemical">nt with hypothesis 1 aclass="Chemical">nd suggested that both P additioclass="Chemical">n aclass="Chemical">nd additioclass="Chemical">n frequeclass="Chemical">ncy sigclass="Chemical">nificaclass="Chemical">ntly iclass="Chemical">nflueclass="Chemical">nced growth of class="Chemical">n class="Species">C. brevicuspis. Moreover, our results confirmed that P enrichment frequency did not significantly influence the biomass allocation of C. brevicuspis, indicating that this was not an effective way for plants to acclimate to different P enrichment frequencies.

The stimulation of plant growth by additional P has been widely reported in other studies (Chiang et al., 2000; McCormick et al., 2001; Mao et al., 2016). For instance, addition of P resulted in a two-fold increase in the biomass of sawgrass and mixed sawgrass-class="Species">cattail commuclass="Chemical">nities iclass="Chemical">n the Everglades Wetlaclass="Chemical">nd, Uclass="Chemical">nited States (Chiaclass="Chemical">ng et al., 2000). Studies have also coclass="Chemical">nfirmed that class="Chemical">n class="Chemical">water lily leaf size was enhanced in response to P enrichment (McCormick et al., 2001; Newman et al., 2004). However, our results contradict those of some other studies (Song et al., 2011; Gao et al., 2016). For instance, Song et al. (2011) found that increased P input had no effect on aboveground biomass of Calamagrostis angustifolia in the Sanjiang Plain Wetland, China, which might have been because plants were adapted to earlier soil P conditions and responded slowly to the addition of P (Macek and Rejmánková, 2007). Another possibility is that P was not a limiting nutrient in the Sanjiang Plain Wetland (Mao et al., 2016). However, our previous study showed that C. brevicuspis was limited by P at Dongting Lake (Li et al., 2017, 2018); this might explain why the addition of P promoted C. brevicuspis growth.

Both foliar and root P concentrations in class="Species">C. brevicuspis were sigclass="Chemical">nificaclass="Chemical">ntly iclass="Chemical">ncreased iclass="Chemical">n the three P additioclass="Chemical">n treatmeclass="Chemical">nts, which might explaiclass="Chemical">n the decrease iclass="Chemical">n C:P aclass="Chemical">nd N:P iclass="Chemical">n these treatmeclass="Chemical">nts. These results were partially coclass="Chemical">nsisteclass="Chemical">nt with our hypothesis 2. Iclass="Chemical">ncreased placlass="Chemical">nt P coclass="Chemical">nceclass="Chemical">ntratioclass="Chemical">n followiclass="Chemical">ng P eclass="Chemical">nrichmeclass="Chemical">nt has beeclass="Chemical">n widely reported (Ostertag, 2010; Soclass="Chemical">ng et al., 2011; Mao et al., 2016). However, study of Eleocharis spp. showed that leaf tissue P coclass="Chemical">nteclass="Chemical">nt decreased with the additioclass="Chemical">n of P, aclass="Chemical">nd caused higher C:P aclass="Chemical">nd N:P iclass="Chemical">n eclass="Chemical">nriched plots (Daoust aclass="Chemical">nd Childers, 2004). These results iclass="Chemical">ndicate that placlass="Chemical">nt P coclass="Chemical">nceclass="Chemical">ntratioclass="Chemical">n respoclass="Chemical">nses to P eclass="Chemical">nrichmeclass="Chemical">nt are species-specific aclass="Chemical">nd might be related to placlass="Chemical">nt absorptioclass="Chemical">n efficieclass="Chemical">ncy aclass="Chemical">nd class="Chemical">nutrieclass="Chemical">nt use strategies, as well as soil microorgaclass="Chemical">nism aclass="Chemical">nd eclass="Chemical">nzyme activity (Rejmáclass="Chemical">nková et al., 2008; Soclass="Chemical">ng et al., 2011; Yuaclass="Chemical">n aclass="Chemical">nd Checlass="Chemical">n, 2015). Moreover, our study coclass="Chemical">nfirmed that both foliar aclass="Chemical">nd root P coclass="Chemical">nceclass="Chemical">ntratioclass="Chemical">ns, C:P, aclass="Chemical">nd N:P iclass="Chemical">n class="Chemical">n class="Species">C. brevicuspis were not significantly different between the three P addition treatments. However, foliar N:P and root P concentration were significantly different, indicating that the influence of P addition frequency on plant stoichiometry was relatively weak. These results contradict our hypothesis 2 and the findings of other studies (Sun et al., 2018). A possible reason is that the amount of P in the three–phase frequency addition treatment was sufficient to support the growth of C. brevicuspis. Another possible reason is that C. brevicuspis has a relatively high stoichiometric homeostasis, which may have maintained stoichiometric stability under different P addition treatments (Han et al., 2014).

Our results suggest that P enrichment decreased foliar N content and increased C:N, which is consistent with the results of other studies (Feller et al., 2007; Yuan and Chen, 2015). The main reason for this finding might be that increased P availability generally stimulates plant growth in P-limited ecosystems, leading to a decline in plant N concentration owing to the dilution effect (Yuan and Chen, 2015; Mao et al., 2016). Another possible reason might be the decrease in soil total N content in the one–time P addition treatment (Table 1). He and Dijkstra (2015) also confirmed that P addition can result in considerable losses of gaseous N from P-poor soils, most likely via direct stimulation of nitrification and denitrification. However, our results differ from those of some other studies (Newman et al., 2004; Mao et al., 2016). In a northern Everglades slough wetland, N concentrations in class="Chemical">water lily geclass="Chemical">nerally iclass="Chemical">ncreased iclass="Chemical">n respoclass="Chemical">nse to iclass="Chemical">ncreased P loads (Newmaclass="Chemical">n et al., 2004), which might have beeclass="Chemical">n due to iclass="Chemical">ncreased soil pore class="Chemical">n class="Chemical">water NH4-N concentrations, mainly from the increased decomposition of resultant nutrient regeneration (Newman et al., 2001). These results suggested that the influence of P enrichment on plant stoichiometry may vary with wetland type and is possibly related to nutrient-limited conditions (Mao et al., 2016).

In conclusion, our results showed that P enrichment promoted plant growth and that this effect increased with decreasing addition frequency. Moreover, we also confirmed that P enrichment, irrespective of the frequency, increased plant C:N and P concentration, but decreased plant N concentration, C:N, and N:P. In recent years, increasing amounts of P have been discharged into Dongting Lake, due to the high intensity of anthropogenic activities. Our results improve our understanding of the influence of P enrichment on wetland nutrient cycling in this lake. However, P input usually occurs with other nutrient elements e.g., N and K. Therefore, the influences of other elements, as well as their interactive effects on plant stoichiometry should be studied to better understand the influence of exogenous nutrient input on nutrient cycling in Dongting Lake.

Author Contributions

FL and CH wrote the manuscript and conducted the technical assays and statistical analysis. YX and WL designed the experiment and edited the manuscript. FL, CH, XC, ZD, and ZH contributed to data collection and interpretation. All authors reviewed the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.