| High-strengthened farmland fertilization has led to a mass inputs of nutrients elements to agricultural downstream rivers since1980s. In order to alleviate such severe situation and improve water quality, many wetlands have been arranged in agriculture area to intercept phosphorus (P) loss from paddy fields. P addition may lead to the decomposition and structural changes of soil organic carbon (C), thus, the influences of external P on sediment C pool should be taken into consideration when wetlands are arranged worldwide for their intercept function, but the mechanism is still largely unknown. Here, we simulated the temporal influences of external P on sediment C pool by integrating gradient P at rates of0(P-0),5%(P-5),10%(P-10),20%(P-20),30%(P-30) and60%(P-60) relative to the initial total phosphorus (TP) content of the sampled sediment, with the purpose to illustrate the role of P on soil dissolved organic matter (DOM) complexity and the biogeochemical cycling of sediment C pool in agricultural wetland. The major experimental results are as follows:Sediment TP and Olsen-P content were strictly followed the loading rates of superphosphate. Microbial biomass phosphorus (MBP) was notably increased (p<0.05). The activity of acid phosphatase (AcP) was significantly suppressed by P addition (p<0.05), few impacts was observed for alkaline phosphatase. Total organic carbon was reduced with a maximum rate of23%, but the generation of dissolved organic carbon (DOC) and microbial biomass carbon (MBC) was stimulated. As for labile components, highly labile organic carbon in P-60increased by64%, recalcitrant organic carbon decreased significantly with a maximum reduction of22%, little impacts was obtained for mid-labile/labile organic carbon. Dehydrogenase made no difference under P treatments, β-1,4-glucosidase (βG) and cellobiohydrolase remarkably increased with P amendment (p<0.05). The release of CO2from wetland sediment and the cumulative mineralization were also enhanced by P loading. The structural characteristics of sediment DOM were another indicator to identify the shifts of soil C cycling. Fluorescence and freshness index increased by a factor of8.5%,64%, respectively. The indices related to sediment humicity decreased with the enhanced P level, except E250/365and E253/203.Pearson analysis had shown an intimate linear dependence between sediment C-P features and DOM structural complexity, illustrating a microbial-derived source for DOM and a reduction of humic components in sediment C pool, which was later reinforced by the clear blue shift in the three-dimensional excitation-emission matrix.Due to the differences of TP concentration existed in solid and liquid phases, TP and dissolved reactive phosphorus of overlying/pore water were elevated with gradient P treatments. DOC concentration was also raised with a factor of64-102%for overlying water and67-105%for pore water, while total nitrogen (N) was not influenced under external P loading.Ecological stoichiometry had been used to analyze C:P of sediment elements, microbial biomass and eco-enzyme activities, as well as N:P in wetland waters. Soil C:P and MBC:MBP decreased regularly under external P inputs, while the trend for lnβG:lnAcP was an opposite one. The three ecological stoichiometry obtained a significant linear relationship, which revealed a stage of organic C decomposition and a transition of DOM structural complexity subjected to P loading. Without regard to the primary production, external P inputs was unfavorable for sediment C sequestration, and the implications of DOM composition may induce a potential C loss in response to global climate change. Additionally, the changeable N:P in wetland waters may also impact the growth of aquatic organisms. |
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