Simulating the effect of crop rotation on soil organic carbon in subsurface drained farmlands in coastal reclamation areas of Jiangsu China

Abstract: The coastal reclamation area of Jiangsu, China is characterized by its flat topography and concentrated rainfall under the monsoon climatic condition. The farmland soils reclaimed from the coastal mudflats have high initial salt content, poor structure and low organic matter content. Improving the soil quality to increase land productivity is very important to ensure agricultural production of the reclamation area. Construction of subsurface drainage system can speed up soil water movement, thus achieve the soil desalination goal more quickly at the initial stages of land reclamation, and later protect crops from excessive water stress during the growing period. But the effect of subsurface drainage on soil organic matter accumulation in the reclamation area remains unclear, the carbon sequestration effect of agricultural measures such as crop rotation and straw returning in subsurface drained fields needs to be explored. Based on the meteorological, soil and crop information of a subsurface drainage experimental base near Dongtai, Jiangsu, China, this study examined the effect of crop rotation and straw returning method on the accumulation process of soil organic carbon (SOC) in subsurface drained fields through combined use of the field hydrological model-DRAINMOD and soil organic carbon model-DNDC (Denitrification-Decomposition Model). The simulation results showed that for the coastal reclamation areas that has shallow groundwater depth, the DNDC model better predicted the accumulation process of soil organic carbon based on the DRAINMOD predicted water table under subsurface drainage. Taking the measured current soil organic carbon content of the study area (2.95 g/kg) in 2021 as the initial value, continued 32 year simulations of different crop rotations by the DNDC showed that, winter wheat and corn rotation combined with full return of straw/stalks to the field significantly increased the soil organic carbon content to 17.85 g/kg; adding alfalfa as a green manure to the rotation produced good carbon sequestration effect due to the increased proportion of activated carbon, the soil organic carbon content was increased to 16.12 g/kg. These results indicate that crop rotations have good carbon sequestration effect, which can gradually build up the soil organic carbon pool in the infertile soil. The simulation results also showed that, comparing with the conventional drainage system of less intensive open ditches in the study area, the subsurface drainage system can lower the field water table more rapidly, and the simulated farmland SOC accumulation is greater due to the increased biomass (or higher crop yields) under better soil drainage condition. In the rainy season of wet years, more frequent water table fluctuations under the subsurface drainage resulted in alternation of soil drying and wetting condition, leading to intensive soil respiration that caused some SOC losses. To avoid the adverse effect of intensive soil respiration on soil carbon loss due to excessive drainage, controlled drainage may be adopted to lower field drainage intensity according to crop drainage requirement. Findings from this research may provide reference to soil quality improvement in the study area and the similar regions for agricultural carbon neutralization.