Data_Sheet_1_Carbon, Nitrogen, and Phosphorus Stoichiometry and Plant Growth Strategy as Related to Land-Use in Hangzhou Bay Coastal Wetland, China.docx

Ecological stoichiometry can not only instruct soil nutrient stocks and availability, but also indicated plant growth strategy and adaptability to environmental changes or stress. This study was carried out to examine the plant–soil Carbon (C), Nitrogen (N), and Phosphorus (P) stoichiometry distributions and patterns in three tidal wetlands [mudflat (MF), native Phragmites australis-dominated community wetland (NW), invasive Spartina alterniflora-dominated community wetland (IW)], and one reclaimed P. australis-dominated community wetland (RW) in Hangzhou Bay coastal wetland. The results showed that land-uses have more effect on C and N contents, and C:N and N:P ratios in plant than in soil, P content and C:P ratios more affected by plant organ and soil depth. Compared to land-use, both plant organ and soil depth have stronger effects on C, N, and P stoichiometry. Among tidal wetlands, plant N content and C:P, N:P ratios were significantly higher in NW than in IW. In contrast, plant C, N, and P contents and C:P and N:P ratios were significantly lower in RW, and plant C:N was higher. Soil C, N, and P stocks were similar between tidal wetlands, and were significant higher than those of RW, indicating that reclamation were not beneficial to soil nutrient storage. In the NW, soil N availability was relatively high, and P availability was relatively low; and leaf N:P was 15.33, which means vegetation was co-limited by N and P nutrients. In addition, plants in the NW mainly adopted a conservative growth strategy, with a significantly low aboveground biomass of 1469.35 g·m2. In the RW, soil N availability was relatively low, P availability was relatively high, and leaf N:P was 3, which means vegetation was limited by N nutrient. In addition, plants in the RW mainly adopted a rapid growth strategy, with a significantly high aboveground biomass of 3261.70 g·m2. In the IW, soil N availability was relatively low, soil P availability was relatively high, and leaf N:P was 5.13, which means vegetation was limited by N nutrient. The growth strategy and aboveground biomass (2293.67 g·m2) of the IW were between those of the NW and RW. Our results provide a reference for nutrient management and evaluating the impacts of land-use types on coastal wetland ecosystems.</p

Soil chemical properties in different soil depths across the 0‒5.2 m soil profiles in the N0 and N600 treatments.

Soil clay content (a), pH (b), soil organic carbon (c) and nitrate content (d) in different soil depths across the 0‒5.2 m soil profiles in the N0 and N600 treatments. N0 and N600 represent fertilizer N input rates of 0 and 600 kg N ha-1 year-1, respectively. Relative errors were less than 0.05 for all the measured parameters (n = 2).</p

Differential immediate and long-term effects of nitrogen input on denitrification N₂O/(N₂O +N₂) ratio along a 0–5.2 m soil profile

High nitrogen (N) input to soil can cause higher nitrous oxide (N2O) emissions, that is, a higher N2O/(N2O+N2) ratio, through an inhibition of N2O reductase activity and/or a decrease in soil pH. We assumed that there were two mechanisms for the effects of N input on N2O emissions, immediate and long-term effect. The immediate effect (field applied fertilizer N) can be eliminated by decreasing the N input, but not the long-term effect (soil accumulated N caused by long–term fertilization). Therefore, it is important to separate these effects to mitigate N2O emissions. To this end, soil samples along a 0–5.2 m profile were collected from a long-term N fertilization experiment field with two N application rates, that is, 600 kg N ha-1 year-1 (N600) and no fertilizer N input (N0). External N addition was conducted for each subsample in the laboratory incubation study to produce two additional treatments, which were denoted as N600+N and N0+N treatments. The results showed that the combined immediate and long-term effects led to an increase in the N2O/(N2O+N2) ratio by 6.8%. Approximately 32.6% and 67.4% of increase could be explained by the immediate and long-term effects of N input, respectively. Meanwhile, the long-term effects were significantly positively correlated to soil organic carbon (SOC). These results indicate that excessive N fertilizer input to the soil can lead to increased N2O emissions if the soil has a high SOC content. The long-term effect of N input on the N2O/(N2O+N2) ratio should be considered when predicting soil N2O emissions under global environmental change scenarios. </p

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Pearson correlation coefficients (r) between soil clay content, pH, and soil organic carbon (SOC) and the immediate and long-term effects of N addition on N₂O emission ratio.

Pearson correlation coefficients (r) between soil clay content, pH, and soil organic carbon (SOC) and the immediate and long-term effects of N addition on N2O emission ratio.</p

Soil N₂O/(N₂O+N₂) emission ratio in different depths across the 0‒5.2 m soil profile in the N0, N0+N and N600+N treatments.

N0 and N0+N represent laboratory anaerobic incubation using the soil N0 and N0+N, respectively. N600+N is the same as the N0+N treatment except for using the soil N600. N0 and N600 represent fertilizer N input rates of 0 and 600 kg N ha-1 year-1, respectively. Bars represent standard deviations of the means (n = 3). Different letters indicate significant difference at p < 0.05 between different treatments.</p

Enhancement of subsoil denitrification using an electrode as an electron donor

Laboratory culture studies have demonstrated that some microbial strains can use electrons generated by electrodes in the denitrification reaction. To test whether the native soil microbiota can use electrode electrons for denitrification, a subsoil slurry was incubated under an electric potential treatment. A potentiostat-poised (−500 mV) electrode served as an electron donor. The electric potential treatment enriches the electroactive denitrifying bacteria and accelerates the nitrate reduction in the subsoil slurry, with N2 as the dominant end product. These results demonstrate that an electrode can serve as an electron donor to enhance the subsoil denitrification. This finding supports the future development of a technique to remove accumulated nitrate in subsoils and reduce nitrate contamination in groundwater

Effects of nitrate and water content on acetylene inhibition technique bias when analysing soil denitrification rates under an aerobic atmosphere

The acetylene inhibition technique (AIT) is the most widely used indirect method of determining denitrification fluxes. Although AIT bias has long been recognised, the contributions of soil denitrification-related parameters to this bias have not yet been well quantified. Using a direct-N2 method as a baseline, we determined the composition of AIT bias and quantified the effects of soil nitrate (NO3−) and water contents on AIT bias under an aerobic atmosphere. The results showed that the AIT severely underestimated the denitrification rate by 5–26 times and failed to capture the dynamic of denitrification with increasing soil water content. The bias increased with increases in the soil NO3− and water contents. Acetylene-catalysed nitric oxide (NO) oxidation accounted on average for 60% (ranging from 31 to 79%) of the bias. The remaining bias was caused by incomplete acetylene inhibition of nitrous oxide (N2O) reduction. Our results indicate that the AIT method is not recommended for denitrification determination under an aerobic atmosphere, especially for soils with high NO3− and water contents

Anthropogenic N input increases global warming potential by awakening the “sleeping” ancient C in deep critical zones

Even a small net increase in soil organic carbon (SOC) mineralization will cause a substantial increase in the atmospheric CO2 concentration. It is widely recognized that the SOC mineralization within deep critical zones (2 to 12 m depth) is slower and much less influenced by anthropogenic disturbance when compared to that of surface soil. Here, we showed that 20 years of nitrogen (N) fertilization enriched a deep critical zone with nitrate, almost doubling the SOC mineralization rate. This result was supported by corresponding increases in the expressions of functional genes typical of recalcitrant SOC degradation and enzyme activities. The CO2 released and the SOC had a similar 14C age (6000 to 10,000 years before the present). Our results indicate that N fertilization of crops may enhance CO2 emissions from deep critical zones to the atmosphere through a previously disregarded mechanism. This provides another reason for markedly improving N management in fertilized agricultural soils. </p