To refine the assessment of topsoil organic carbon (SOC) sequestration potential at the regional scale and provide scientific underpinnings for targeted carbon sink management and the achievement of agricultural carbon neutrality, Jiangsu Province was selected as a typical study area in this work. Based on 1,285 topsoil samples collected between 2006 and 2008, an integrated approach combining spatial inversion and local spatial autocorrelation analysis was adopted to systematically identify the spatial clustering patterns and distribution characteristics of SOC in cultivated land. After clarifying the spatial distribution features, an overlay analysis of land-use types and SOC hot/cold-spot clusters was performed to delineate SOC-homogeneous subregions and quantify their respective Carbon Sequestration Potential Indices (CSI). Based on these results, functional zoning for carbon sinks was conducted using the natural breakpoint method. Finally, the soil carbon sequestration potential of dryland in 15 counties of northern Jiangsu located within the carbon sink enhancement zones was estimated under a "conversion of dryland to paddy field" scenario, providing spatial support for precise carbon sink management. The results showed that: (1) The spatial distribution of SOC in the cultivated topsoil of Jiangsu Province exhibited a distinct south–north gradient, with high values in the south and low values in the north. The Taihu Lake Basin in southern Jiangsu was identified as the core hot-spot cluster with high SOC concentrations, benefiting from favorable hydrothermal conditions and well-established farming systems that enhance soil organic carbon accumulation. In contrast, northern Jiangsu and the coastal areas formed contiguous cold-spot regions with low SOC contents, primarily influenced by soil texture, climatic conditions, and farming practices. (2) The CSI of cultivated land across the province ranged from 0.33 to 0.58, with significant spatial variation. Notably, several dryland areas in northern Jiangsu exhibited the highest CSI values, making them the key zones for improving regional carbon sink capacity. This is closely related to the optimization potential of soil conditions and land-use practices in these areas. (3) Based on the quantitative CSI assessment, cultivated land in Jiangsu was divided into three functional zones: the carbon sink enhancement zone (located in the lower reaches of the Huai River in northern Jiangsu and the coastal tidal flat areas), the carbon sink consolidation zone (concentrated in the Taihu Lake Basin and the central Jiangsu plains), and the carbon sink regulation zone (transitionally distributed across central Jiangsu, the Ning-Zhen-Yang hilly areas, and the Yellow River floodplain ridges in northern Jiangsu). This zoning framework clarifies the priority directions for carbon sink management in each region and provides a solid spatial basis for implementing "one policy per zone" and targeted management strategies for cultivated land carbon sinks. (4) Scenario simulation results indicated that converting 832,600 hectares of dryland in 15 counties of northern Jiangsu—located within the carbon sink enhancement zone—to paddy fields would result in a net increase of 12.60 million tons in topsoil SOC storage, with increases exceeding 75% in some subregions. This finding confirms that rational conversion of cultivated land use types is an effective approach to enhancing regional carbon sink capacity, primarily due to the flooded environment of paddy fields inhibiting SOC decomposition and promoting carbon accumulation. Overall, the closed-loop research framework established in this study—comprising spatial identification, potential assessment, functional zoning, and scenario validation—features rigorous logic and strong practical applicability. It not only refines the technical approach for large-scale cultivated land carbon sink assessment but also offers a robust scientific paradigm and practical reference for the precise management of regional cultivated land carbon sinks and the advancement of agricultural carbon neutrality.