Opposite effect on soil organic carbon between grain and non-grain crops: Evidence from Main Grain Land, China

The soil organic carbon (SOC) content of cropland affects global food production and is crucial for agricultural carbon reduction. The SOC response to long-term changes in main grain land (MGL) likely differs from non-MGL cropland. We constructed a quantitative indicator system for the MGL planting pattern to reveal the relationship between MGL use change and SOC. We produced an MGL dataset for China from 1985 to 2020 based on the existing MGL distribution data and available Landsat images and analyzed the heterogeneity of SOC under different MGLs using two SOC datasets. Time series change detection explained the interaction between planting patterns and SOC. Different MGLs exhibited considerable heterogeneity in SOC. The single rice area with the highest SOC was twice (31.62 g/kg) that of the wheat and maize areas. SOC showed an opposite trend in MGL and non-MGL with increased planting intensity. Soil SOC content affected the expansion decisions of farmers in cropland and MGL. Areas with a high SOC were developed into MGL by farmers early. MGL consumed more SOC and had a stronger urgency for fallow cultivation than non-MGL. The planting intensity of MGL more than the inflection threshold (about 15 years) decreased shallow SOC (0–15 cm), decreasing below the initial value after more than 30 years. Currently, 62.08 % of MGL in China causes a decrease in SOC, with an urgent need for a fallow MGL of 27.38 Mha. Theoretical guidance for optimizing crop planting patterns to ensure food security and reduce agricultural carbon emissions was provided.