| As frequently disturbed and regulated by anthropogenic impacts in short periods, agricultural ecosystem plays a key role in carbon (C) sequestration and greenhouse gasses (GHGs) reduction. Soil nutrient nitrogen (N) and phosphorous (P) feature is highly related to global soil C cycling with respect to agricultural nutrient management. As one of the essential fertilizer, excessive P fertilization may lead to P loss and the subsequent water eutrophication. Thus, it is necessary to evaluate the role of P fertilization on C and N biogeochemical processes, which enhanced our ability to predict nutrient-related soil C and N cycling in the context of climate change. Here, in2011and2012, we collected samples from a field experiment after7years of superphosphate application with a gradient of0,30,60,90kg P hm"2a-1(P-0, P-30, P-60, and P-90, respectively) in order to evaluate the role of exogenous P on the C and N biogeochemical processes, ecosystem services and their feedback effects, which could clarify P-C-N ecological stoichiometry driven by the ecological and microbial-mediated genetic mechanisms in paddy field ecosystem. The main results are as follows:(1) Generally, soil carbon (C), nitrogen (N) and phosphorus (P) pools had significant increased when receiving P fertilizer. Soil total organic carbon (TOC) and dissolved organic matter (DOC) significantly (p<0.05) increased by1-12%and12-43%following P fertilization, respectively, as well as the highly labile organic carbon, mid-labile organic carbon and labile organic carbon. Soil total N (TN) also significantly (p<0.05) increased by3-24%under P fertilization, compared to control treatment (P-0), except for P-30in Novermber. Soil total P (TP) significantly (p<0.05) increased with the increasing P application by16-153%, which have resulted in significant P accumulation in our paddy field soil. Meanwhile, a similar trend was observed for soil Olsen-P. Specifically, Olsen-P content under P-60treatment was closed to0.32mmol kg-1as a threshold, implying fertilizer-P application at60kg hm-2was suitable for rice yield. The P fractionation scheme revealed that P fertilization significantly shifted different P fractions. Except for a few soil samples, P application significantly (p<0.05) enhanced the NH4C1-P, BD-P, NaOH-TP, HC1-P and NaOH-DP content, but the opposite trend occurred for NaOH-OP and Res-P. Therefore, P application not only increased soil TOC/TN/TP pools, but also the labile and bioactive soil C and P pools, which is beneficial for the enhanced soil fertility levels and soil productivity.(2) It is obviously that soil microbial biomass (MBC/MBN/MBP) significantly changed subjected to different sampling time, and P fertilization significantly enhanced the soil microbial biomass. The Î'-glucosidase, which was involved in C cycling, showed the positive relationship with P application, while the N-related and P-related enzymes of NAG+LAP and AP showed the negative relationships. The first-order kinetic analysis of cumulative carbon mineralization showed that P input enhanced the mineralization rates of organic carbon, which contributed to the higher C cycling. The spectroscopic characteristics of DOC suggested that P input induced the enhancement of small molecular compounds, less humicity, and decrease of aromatic compounds, which led to more labile C sources for microbial acquisition, and accelerated DOC consumption. However, P fertilization also accelerated the generation of labile C sources and contribute to the higher input of microbially-derived DOC. Combined these two increased features of soil DOC production and decomposition above, P fertilization accelerated DOC cycling, which contributed to accelerated C cycling. Notably, although the DOC cycling was accelerated, the DOC content also significantly (p<0.05) increased, contributing to a net DOC sink under P fertilization. Therefore, integrating the increased levels of TOC and labial C with DOC content, P fertilization may lead to soil C sequestration in paddy field ecosystems. Moreover, significant (p<0.05) correlations between P-related characteristics and C-related and N-related properties also verified that soil P pool properties were key factors of C and N pool subjected to P fertilization.(3) The real-time quantitative PCR technology showed the abundances of bacteria and fungi, and the ratio of fungi to bacteria relatively increased following P input, perhaps suggesting a mechanism for the decline in DOC structural complexity so as to increase the bioavailability of DOC and contribute to an enhancement of microbial-derived DOC. However, P amendments decreased the abundance of soil archaea and accordingly reduced the corresponding ratio of archaea to bacteria. Using454pyrosequencing, compared to P-0and P-30, P-60and P-90consistently and significantly (p<0.05) increased bacterial species richness and diversity (Ace, Chao and Shannon). The16S rRNA genes profile revealed that P fertilization shifted the structure of their bacterial communities with respect to the differences detected in cluster analysis, while P-60showed the highest divergence from the other three treatments. This mainly because that the patterns of bacterial community composition was significantly influenced by the biochemical properties following P input throught regulating trophic micro-environment in soils. Specifically, P application enhanced8species of bacteria at order level belonging to Proteobacteria, as well as Propionibacteriales, Addimicrobiales, Cytophagales and Gemmatimonadales. P fertilization induced Oceanospirillales, Campyiobacterales, Propionibacteriales growth, all of which were undectable in P-0. Additionally, P fertilization also enhanced the abundances of Methylophilales and Rhodocyclales, which may act as a major mechanism underlying P-induced consumption of CH4as a way of mitigating the greenhouse effect.(4) The stoichiometric ratios of soil C:P and soil N:P declined significantly (p<0.05) with increasing P application. However, no significant differences were found for the soil C:N ratio among the four treatments in May, but such ratio significantly declined in August and November. P input induced C and N limitations as indicated by the decreased ratio of C:P and N:P in microbial biomass due to the declined soil C:P and N:P. A synergistic mechanism among the negative relationship between ecoenzymatic stoichiometry (lnBG:In(NAG+LAP):lnAP) and C:N:P in microbial biomass regulated the ecological function of microbial C and N acquisition, which were stoichiometrically related to P input and stimulated soil C and N sequestration in the paddy field soils.(5) The effects of P fertilization on C, N, and P contents in the overlying water and P content in porewater were measured. P input significantly enhanced the DOC content in overlying water. Except for P-30treatment in August and September, TN in overlying water was also increased under P fertilization. Both in overlying water and porewater, P fertilization almost enhanced the concentrions of TP, DRP and TPP in most of sampling times, as well as the potential P fluxes (Fp) compared to the control treatment (P-0). Previous studies showed that when TN/TP ratio in water was less than7, N was the limited element for algae growth. In our study, TN/TP ratio was less than7, and even less than4in Aug and Sep under different P applications, which may lead to N limitaiton and was not suitable for algae growth. In paddy field, obvious seasonal changes occurred in CH4fluxes under P fertilization, which initially increased, and then decreased, followed by a minor increase again at the stage of maturity. P fertilization remarkably declined the CH4fluxes in rice-growing stage with minimum values occurred both in P-60and P-90. However, compared with P-0, P fertilization remarkably promoted CO2fluxes in rice-growing season. Except for heading stage, N2O fluxes reduced under P fertilization, but P-60and P-90contributed to the lower N2O fluxes than P-30treatment. Therefore, P management, as a static entity, may offer an advantageous technology for controlling CH4and CO2fluxes in paddy field ecosystem, but there still remains inadequate studies on CO2fluxes. Due to P application, soil C pool properties and P content had positive effect on CO2fluxes, but negative effect on N2O and CH4fluxes. Moreover, N2O and CH4fluxes had positive effect on microbial biomass stoichiometric ratios. Taken together, these results suggested that the soil C pool properties and microbial biomass stoichiometric ratios could be used for simultaneously regulating three types of GHGs in terms of the eco-stoichiometry mechanism under P fertilization.(6) Phosphate fertilizer is the dominant factor in rice growth, beneficial for the gain of rice yield. P input significantly (p<0.05) increased rice grain yield and1000-grain weight by factors of22-47%and7-12%, respectively. The total grain per panicle was solely increased by9%under P-60treatment, but no significant effect on total grains per panicle under other treatments. Co-inertia analyses showed that paddy field ecosystem was significantly influenced by P fertilization and different samping seasons, and it showed the similar trend among the same season. Judging from the integrated scores of the both principal components, the highest soil biological fertility displayed in P-60, then in a decreasing order of P-90, P-30, and P-0. Thus, it is recommended that the P input in paddy fields not exceed60kg hm-2may maximize soil C sequestration, minimize P export, and guarantee grain yields. |
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