Effect of intensive farming practice on soil organic carbon stock in Huantai County

Agriculture is an important source of greenhouse gas (GHG) emissions and an important approach for carbon stock enhancement in terrestrial ecosystems. Optimization of farming practices can not only improve the utilization efficiencies of chemical fertilizer, but also increase the soil organic carbon (SOC) and hence improve soil fertility with the prerequisite of high and stable grain yield, and mitigate the greenhouse effect via increased SOC. The study chose the intensive farmland of Huantai County in Shandong Province, the first county to achieve 1000 kg of grain per mu (15 tons per hectare) in North China, to analyze the impacts of farming practices including fertilization, organic manure application, and crop straw incorporation, on SOC stock in the whole county. A biogeochemical model, DeNitrification-DeComposition (DNDC), was validated at the plot and regional scales for its applicability to SOC simulation. Five scenarios of farming practices were set, with full consideration of the natural and social conditions of Huantai County. With 2011 as the baseline year, the SOC changes in farmland soil within 30 years after 2011 were simulated. The simulation using the DNDC model showed that, under the scenario of maintaining the farming practices of 2011 (annual average chemical fertilizer of 500 kg(N)∙hm⁻² and 90% crop straw incorporation), the SOC content increased by 28.1%, 39.2%, and 44.9%, respectively, at 10, 20, and 30 years after 2011. This indicates that there is still room for soil organic matter to increase under current climate conditions, mainly due to crop straw incorporation in Huantai County. However, the increase in SOC is not linear, the increasing rate declines over time, and SOC saturation will occur for the farmland soil in Huantai County. Among the five farming practice scenarios, an annual average chemical fertilizer application rate of 400 kg(N)∙hm⁻², straw incorporation ratio of 90%, and animal manure application rate of 40 kg(N)∙hm⁻² can maximize farmland SOC content, reaching 16.2 g∙kg⁻¹ at 30 years after 2011. These results can provide a scientific basis for evaluating the farmland SOC pool in the North China Plain and provide technical support for the optimization of farming practices and policy improvement.