Soil organic carbon dynamics (2008–2020) across different cultivated land use types in a subtropical region

Soil organic carbon (SOC) improves soil structure, retains water, and increases nutrient availability, which enhances crop productivity and serves as a significant carbon sink for climate mitigation. While it has been proposed that alterations in land use can significantly influence SOC sequestration, limited research has delved into the distinct impacts of various types of cultivated land use. The main objective of this study was to compare differences in SOC dynamics across diverse cultivated land use types in 84 counties (cities and districts) within Fujian Province, a typical subtropical area of China. This study utilizes 230,445 measured soil samples and a high-resolution 1:50,000 soil map—the most detailed database of its kind applied at the provincial scale—to simulate SOC dynamics under different cultivated land use types from 2008 to 2020. By integrating this comprehensive dataset with the DNDC (Denitrification and Decomposition) model, our approach offers greater spatial precision and more robust simulations. The results indicated that the average annual soil carbon sequestration rate (d_SOC) for all cultivated land, paddy fields, dry land, and irrigated land, was 13.16, 30.98, −12.31, and −5.52 kg·ha⁻¹, respectively. The total change of SOC (T_ΔSOC) values were 171.06 Gg and 402.80 Gg for all cultivated land and paddy fields, while they were −160.20 Gg and −71.72 Gg for dry land and irrigated land, respectively. Over the past 13 years, the interannual variations of T_ΔSOC reveal a range of −263.66 to 254.71 Gg across all cultivated land in the province. Positive T_ΔSOC for all cultivated land dominated from 2008 to 2016, while a shift to negative values is apparent after 2017. In general, the cultivated land throughout the province displayed a modest carbon-sink behavior. More specifically, paddy fields functioned as effective carbon sinks, while both dry land and irrigated land behaved as carbon sources. The high initial SOC content is the main direct factor negatively correlated with d_SOC. Rainfall indirectly reduces d_SOC by promoting an increase in clay content in both dry land and irrigated land. To improve future agricultural management, we should increase the amount of organic fertilizer applied to paddy field while maintaining economic efficiency and crop growth, and apply biochar and implement wet season drainage for both dry and irrigated land.