The Synergistic Effects Of Evolution Of Soil Organic Carbon On Increment Of Crop Yield In Arable Land In China

Soil organic carbon(SOC) is one of the most important indicators of agroecosystem, and plays important role in increasing crop productivity and improving soil fertility. The quanlity and health of soils can be controlled by processes of SOC cycling. However, the evolution of SOC and its impacts on crop yield has not yet been clearly quantified.Thus, 17 well managed long-term experiments with different climate, soil types and cropping stsyems in China were chosen to 1) quantify changes in SOC stock and the relationship between SOC stock and crop yield, 2) evaluate SPACSYS model on modelling storage of SOC and nitrogen, crop yield and processes of C and N cycling, and 3) predict the future changes in crop yield and SOC stock under different fertilization with different climate conditions, and analysis the effects of SOC on crop yield in the future under different climate conditions. The main results and conclusion were: 1) Annual application of inorganic fertilizers(NPK) plus animal manure over 20-30 years significantly increased SOC stock(20-cm depth) by 32-87% whilst NPK plus wheat or maize straw application increased SOC stock by 26-38% compared to the non-fertilized controls. In the northern regions, application of manure had relatively little effects on yield compared to NPK when topsoil SOC stocks were increased to 18- 50 Mg C ha-1. In the South, however, average yield from manure application treatments was 2.5 times greater than that from NPK treatment, and relative yields from manure application treatments were not increased until SOC stocks(20-cm depth) were more than 35 Mg C ha-1(1.39% Dry matter). In the northern regions, yield stability was not increased by application of NPK plus manure compared to NPK, whereas in the South there was a significant improvement. We conclude that manure application and straw incorporation could potentially lead to SOC sequestration in topsoil in all typical agricultural regions of China, but beneficial effects of this increase in SOC stocks to 20-cm depth increase on crop yield and yield stability may only be achieved in the South. 2) The SPACSYS model can simulate the dynamics of SOC and SN and yields of winter wheat and summer maize from different fertilization treatments, especially for the treatments with application of chemical fertilizes plus manure, and straw incorporation. The simulated wheat and maize yields were significanltly correlated with observed value(R2>0.8). In addition, simulated values of SOC and total nitrogen were significant correlated with observed value(R2>0.81) and the RMSE were 3.12% and 2.61%, respectively. Furthermore, soil CO2 and N2 O emissions were well simulated indicating the SPACSYS can be used for modelling soil C and N cycling. However, some discrepancies between simulated and measured value can not be fully avoided like the other models. On the whole, SPACSYS model is suitable for modelling soil carbon and nitrogen in arable land in China. 3) Base on four climate scenarios(Baseline, RCP2.6, RCP4.5 and RCP8.5), predicted results indicated that application of manure(NPKM and hNPKM) can enhance wheat yield by 20-30% under the rotation of double-cropping until 2100, and by 10-20% under rotation of mono-cropping in individual climate scenario. Maize yield can be increased by 10-25% in NPKM and hNPKM treatments under mono- and double-cropping systems. Furthermore, application of manure and straw incorporation significantly increased SOC stock with the increasing rate of 0.237-0.360 t C ha-1 yr-1 compared to unfertilized treatment under either mono-cropping or double-cropping. In addition, application of manure led the increment of 44-86% of SOC stock compared to NPK treatment, but straw incorporation only increased SOC stock by 31-33% compared to NPK treatment under double-cropping system. The results from linear-plateau model indicated that C input from manure and starw can increase the maximal wheat and maize relative yield by 14-17% and 18-20% compared with NPK. Under all climate scenarios, the threshold values of SOC on crop yield were not obviously changed(~23 t C ha-1 on average from 1989-2005). Further, SOC stock was not related to wheat and maize yield(R2=0.056,P>0.05) in the mono-cropping regions. After application of manure, C and N losses per 1 kg grain yield were the lowest among treatments, and were 20-46% and 2-24% lower than those in NPK treatments among regions. Thus, application of manure can sustain soil fertility and alleviate the negative effects of agricultural practices on climate change. However, application rate of manure and combine ratio of NPK and manure needs to be further optimized.