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Understanding the spatiotemporal changes in soil organic carbon (SOC) and their driving factors is an important prerequisite in decision-making for maintaining sustainable agricultural development and addressing climate change. However, the impacts of urbanization-induced changes in cropland management practices on the spatiotemporal changes in SOC and their implications remain unclear, in particular over different time periods. In this study, a total of 1219 cropland topsoil SOC data (0-20 cm) were collected from a rapidly urbanizing area of southeastern China (southern Jiangsu Province) in 1980, 2000, and 2015, and geostatistical sequential Gaussian simulations 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 À1 during the period of 1980-2000 and a net decrement of 2.32 g kg À1 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 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)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(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.