Understanding the spatiotemporal changes in soil organic carbon (SOC) and their driving factors is an important prerequisite for decision-making in maintaining sustainable agricultural development and addressing climate change. A total of 1219 cropland topsoil SOC data (0-20 cm) collected from southern Jiangsu Province of China in 1980, 2000, and 2015, and geostatistical sequential Gaussian simulation were used to identify the changes in the spatiotemporal patterns of SOC during the period of 1980-2015. Results showed that the changes in SOC within the different time periods were significantly different, with a net increment of 3.65 g kg-1 during the period of 1980-2000 and a net decrement of 2.32 g kg-1 during the period of 2000-2015. Significant SOC accumulation occurred throughout the study area during 1980-2000, while SOC decline became predominant in the southeast during 2000-2015. Overall, the SOC contents for 60% of the study area increased significantly over the entire 35-year period. The SOC increase during the first two decades (1980-2000) was largely attributed to the increasing soil C input that resulted from the enhanced crop productivity by chemical fertilizers, while the stagnant soil carbon inputs associated with the rapid urban expansion were the primary reason for constraining cropland SOC accumulation in the subsequent 15 years (2000-2015). These findings highlight the importance of balancing agricultural development and urbanization processes to maintain SOC levels, and may also provide some guidance for planning cropland soil C management strategies in many areas that are undergoing similar urbanization processes.