Modeling Soil Organic Carbon Dynamic Changes In Organic Farming

Global warming is a great concern, carbon sequestration and dynamics in soil subjected organic farming was studied and modeled using culture experiments, field testing, and located monitoring in this research. The following aspects were focused on:1. The decomposition characteristics, fractions of readily decomposed components, and their effects on soil organic carbon mineralization of three organic fertilizers;2. determination of parameters of Soil-C model, verification and application of Soil-C model in soil subjected organic farming;3. determinaiton of the best formulation of organic fertilizers and their optimal application ways using simulated results of Soil-C model.Dynamic changes of organic carbon in commercial organic fertilizer (SP), rapeseed cake fertilizer (BF), and straw compost (DF) under culture conditions were studied in soil respiration bottles for90days. It was found that the decomposition of three organic fertilizers showed different characteristics at culture conditions. The decomposition rate of rapeseed cake fertilizer and straw compost underwent a rapid increasing stage which was followed by a quick declining during the first25days after they were mixed with soil. Such a trend was more significant in rapeseed cake fertilizer with lower C/N ratio. Nevertheless, the decomposition rate of commercial organic fertilizer which has higher C/N ratio than the others was lower. Total amount of mineralized organic carbon in soil with different fertilizer treatment followed the sequence of rapeseed cake fertilizer> straw compost> commercial organic fertilizer, with statistical significance of P<0.01. The data were fitted to the Soil-C model and the first order kinetic constants K1, K2, Ks were determined to be0.019,0.97×10-3, and0.80×10-3(d-1), respectively. Meanwhile F(C)-the percentages of CO2-C released in commercial organic fertilizer, rapeseed cake fertilizer, and straw compost were0.22,0.57, and0.47, respectively. The comparison between experimental data and model simulation revealed that mixing commercial organic fertilizer with rapeseed cake fertilizer at ratio of2:1significantly enhanced its decomposition (P<0.01). Similar enhancement was also observed when straw compost was mixed with rapeseed cake fertilizer at ratio of1:1(P<0.01).The results in culture experiments showed that Soil-C model can accurately describe the dynamics of soil organic carbon. However, in order to make sure this model could be applied in field with organic farming and deduce the calculation formula of the fraction of readily decomposed components of the fertilizers, this model was further verified in field test. It was found that the decomposition characters of three organic fertilizers in field test were equal to those in culture experiments. The results of open-air field trials in different seasons showed that the samples of three fertilizers which buried in autumn decomposed faster than those buried in spring during early days. However, they decomposed slower during the later period. This indicated that environmental factors such as temperature effected the decomposition of organic fertilizers significantly. Such a trend was also observed in field trials in the greenhouse. The decomposition rate of commercial organic fertilizer was slow in winter but it became fast in the following spring. The results in field trials in the greenhouse also showed that in the first6months, the decomposition rates in commercial organic fertilizer, rapeseed cake fertilizer, and straw compost accounted for70%,91%, and80%, respectively, of the annual decomposition. As a result, the dynamic of organic fertilizer could be fully reflected in half a year observation. Comparing to F(C), F(w)-percentage of organic carbon loss was closer to the F(A) and F(S)-the actual percentage of decomposition. Hence, F(w) was more suitable to represent the fractions of readily decomposed components in three fertilizers. Good correlation between measured data and model simulation could be constructed, which revealed that the Soil-C model was suitable for simulating soil organic carbon dynamics under organic farming.Finally, results of located monitoring in different farming system showed that both microbial biomass C and organic matter increased significantly in soils subjected organic farming practice for10years (OR) and6years (ZH), due to the extensive use of organic manure and reasonable agronomic management practices, Nevertheless, similar change was also happened in conventional farming system in the early years but not sustained later. The measured soil organic carbon in OR and ZH were consistent to the simulated values, Improved fertilization practice was adopted according to Soil-C model prediction. Comparing to using a single organic fertilizer, mixing commercial organic fertilizer with rapeseed cake fertilizer could increase the fertilization efficiency, although soil organic carbon content had no significant difference among the three methods of fertilization.