Study On The Effect Of Crop Rotation On Soil Organic Carbon Accumulation In Subsurface Drainage Fields In Coastal Reclamation Area Of Jiangsu,China

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 shallow groundwater depth,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.Rotation is a widely used measure in agriculture to improve farmland productivity and soil fertility.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.Therefore,based on some experimental data of a subsurface drainage experimental base near Dongtai,Jiangsu,China,and referring to the standard of soil waterlogging and drainage in coastal reclamation area,this study simulated the change of groundwater depth dynamics under different subsurface pipe arrangement by using the field hydrological model-DRAINMOD.Then,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 DRAINMOD model and soil organic carbon model-DNDC(Denitrification-Decomposition Model).Findings from this research may provide reference to soil quality improvement in coastal reclamation areas and the similar regions for agricultural carbon neutralization.The main research achievements in this study are as follows:(1)For the coastal reclamation areas that has shallow groundwater depth,the field hydrological model-DRAINMOD can accurately simulate the change of groundwater depth in the subsurface drainage field,and get the effect of different subsurface drainage conditions on farmland waterlogging according to long-range meteorological data.The soil organic carbon model-DNDC better predicted the accumulation process of soil organic carbon based on the DRAINMOD predicted water table under subsurface drainage.(2)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.(3)The simulation results 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.However,with the abundant rainfall in rainy season,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.(4)In the case of no artificial irrigation and long-term crop rotation with straw returning,the arrangement of subsurface tube with a depth of 1.2 m and a distance of 10 m had the best soil organic carbon accumulation effect in 9 simulated layouts,which had the best mitigation effect on SOC loss in rainy days,and also had the best assurance rate of rainfall.In order to avoid soil organic carbon loss caused by frequent rainfall in rainy season,and combine with waterlogging reduction and crop yield improvement,it is recommended that the buried depth of the dark tube be greater than 0.9 m and the spacing be less than 25 m.