Dynamics and Rates of Soil Organic Carbon of Cultivated Land Across the Lower Liaohe River Plain of China over the Past 40 Years

The Lower Liaohe River Plain (LLRP) is a core grain production base in Northeast China. Monitoring the dynamics and changing rates of soil organic carbon (SOC) in cultivated lands is essential for regulating soil fertility, safeguarding food production, and maintaining the regional carbon balance. Based on soil survey data from three periods, 1980, 2008, and 2019, this study investigated the spatiotemporal dynamics of SOC content and its changing rate (SOCr) using geospatial analysis. Results showed that SOC content declined significantly from 11.19 g kg−1 to 10.47 g kg−1 during 1980–2008, then recovered slightly to 10.58 g kg−1 in 2019. Moreover, SOCr varied temporally in the period of 2008–2019, exhibiting a positive mean rate of 0.01 g kg−1 yr−1, which was significantly higher than that of the period of 1980–2008 (−0.03 g kg−1 yr−1). A significant negative correlation was examined between the initial SOC content and SOCr, showing an identification of the SOC equilibrium point (SOCep). The SOCep in the period of 2008–2019 was 9.69% higher than that in the period of 1980–2008. These findings provide a scientific basis for formulating regional policies and optimizing spatially differentiated management strategies to improve cropland SOC in the study area.