| Carbon (C) cycle and C sequestration efficiency in paddy soil depend on the key nutrient elements input and availability, such as nitrogen (N) and phosphorus (P), and C:N:P stoichiometric ratio. Soil microbes use nutrient elements by ecological stoichiometry, which also influences C cycle and C sequestration efficiency. Therefore, we did research on the coupling relationship between C turnover and nutrient elements input in paddy soil, in order to clarify the impact of N, P input to C turnover and reveal the mechanism of ecological stoichiometry controling upon C turnover, which is beneficial in improving the production of paddy soil, enhancing the potential of carbon sequestration and controlling the greenhouse gas emissions.In this study,13C and 15N double labelled microcosm experiment was carried out under different elemental stoichiometric ratios. We added 13C-glucose as carbon source,15N labelled (NH4)2SO4 as nitrogen source and NaH2PO4 as phosphorus source to a typical subtropical paddy soil. Set treatments as follows, soil (CK), soil with glucose (Glu), soil with glucose and N (Glu+N), soil with glucose and P (Glu+P), soil with glucose and N, P (Glu+NP). The main results were as follows:(1) The glucose-C fate in paddy soil included three pools:1) mineralized to CO2, CH4, 2) remained in soil solution as DOC and 3) incorporated into SOC. After 100 days incubation,58.6%～63.6% of added glucose-C was mineralized to CO2 and CH4, the level of mineralization rates among treatments was Glu+N>Glu+NP>Glu>Glu+P. The second pool, remained in soil solution as DOC, were almost none in all treatments. 14.6%～16.3% of glucose-C was immobilized in soil eventually. Meanwhile, adding exogenous C had a positive priming effect on the soil original organic matter, and nutrient elements input enhanced the positive priming effect.(2) The exogenous N paddy soil transformed rapidly. Compared with 5 days incubation, after 100 days incubation, in Glu+N treatment, the percentage of NH4+-15N in soil solution decreased from 10.7% to 4.22%,15DON in soil solution kept the same level, NH4+-15N in soil increased by 4.41%, and 15MBN changed from 16.5% to 8.01%,15DON in soil increased by 2.17%. Nevertheless, in Glu+NP treatment, the percentage of NH4+-15N in soil solution decreased from 7.72% to 0.03%, 15DON in soil solution increased by 2.97%, NH4+-15N in soil changed from 38.4% to 20.7%,15MBN increased by 15.5%,15DON in soil stayed the same level. Glu+P and Glu+NP treatments had higher Olsen-P concentrations than other treatments because of exogenous P input, Olsen-P and MBP levels decreased as time passed by.(3) The substrate concentrations and activities of β-1,4-glucosidase (β-Glu), β-1,4-N-acetylglucosaminidase (N-Ac) and phosphatase (Phos), which were involved in C, N and P cycles in paddy soil, respectively, were well fitted with Michaelise-Menten model. Compared with the CK, β-Glu Vmax of each treatment increased by 1.88%～36.1%, while N-Ac Vmax increased by 8.64%～62.3%. The activity of β-Glu was significantly positive correlated with the activity of N-Ac (R2=0.71, p<0.01). However, treatments with P made Phos Vmax lower than no P treatments, increased activities of β-Glu and N-Ac.(4) Adding C, N and P into paddy soil changed C:N:P ratio of soil, microbial biomass and enzyme activities. We can see exogenous C transformation was influenced by mutiple factors by multivariate analysis. Nutrient addition had direct effects on soil physical and chemical environment, enzyme activities and so on, and then influenced exogenous C turnover directly or indirectly. |
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