Current research on soil organic matter (SOM) in the black soil region has primarily focused on the overall decline in SOM content, while the dynamics of its spatial heterogeneity have received far less attention. In fact, SOM spatial heterogeneity not only reflects the degree of equilibrium in soil quality distribution but also serves as a critical dimension for evaluating cropland degradation and carbon sequestration potential. To address this gap, this study targeted the black soil region of Northeast China and, based on 2810 topsoil samples (0-20 cm), multitemporal satellite remote sensing data (Landsat series), and a random forest regression model, constructed a high-resolution SOM inversion dataset at 30 m resolution-NEChina-SOM-30 m-Decadal-covering four decadal intervals (1984-1993, 1994-2003, 2004-2013, and 2014-2023), while accounting for differences between paddy and dryland. The inversion dataset enabled the spatiotemporal reconstruction of cropland SOM content across the region. Model validation (R2 = 0.634, RMSE = 6.471 g/kg) demonstrated the stability and reliability of the proposed framework. Using a 5 km grid-based approach, we systematically analyzed the temporal dynamics of SOM content and its spatial heterogeneity, measured by the coefficient of variation (CV), and further revealed regional divergence patterns under a four-partition framework (Northeast, Central, Southwest, and Northwest). Results indicate that SOM in Northeast China exhibited pronounced dynamic changes, with 52.5 % of cropland showing a declining trend, averaging a decrease of 5.12 g/kg. Notably, areas with intensified spatial heterogeneity accounted for 78.6 % of the cropland, substantially exceeding the extent of SOM-declining areas, implying a trend toward increasingly complex and uneven soil spatial structures. Moreover, 38.9 % of cropland simultaneously experienced SOM loss and enhanced SOM spatial heterogeneity, highlighting a compounded effect of regional cropland degradation. Subregional analysis further revealed that cropland in Heilongjiang Province exhibited the largest magnitude of SOM change with a clear increasing trend, while Jilin Province showed the highest proportion of SOM decline and the most pronounced expansion of its spatial heterogeneity. This study provides the first regional assessment of the coupled evolution of SOM content and spatial heterogeneity in the black soil region. The results highlight spatial heterogeneity as a key indicator of soil system stability and cropland degradation, offering a practical framework to support soil conservation, sustainable land use, and precision management.