Associations of Land Use and Conservation Measures with Soil Organic Carbon in Small Watersheds of the Black Soil Region of Northeast China

Soil organic carbon (SOC) is a critical indicator for soil fertility and ecosystem stability, yet its spatial distribution in slope croplands remains poorly understood at the small watershed scale. This study compared SOC content and its associated factors in two small watersheds in the hilly black soil region of Northeast China that share similar topographic and climatic conditions but differ in the adoption of slope cropland conservation measures. Surface soil samples (0–20 cm) were collected across both watersheds using a 400 m × 400 m grid, and a random forest model was used to identify the association of each factor with SOC content. The SOC content in forests was higher than that in cropland. The SOC content in cropland with soil and water conservation measures was 51.84% higher than that in cropland without such measures. SOC tended to be lower on steeper slopes. For slopes of 5~8°, cropland with conservation measures had SOC 76.98% higher than cropland without measures. Random forest analysis showed that the relative importance of SOC predictors varied among different management types. The contribution of total nitrogen (TN) and slope was higher on cropland without conservation measures, while the importance of soil pH and slope was relatively higher in cropland with measures. These findings suggest avenues for research to optimize conservation measures to improve soil quality in black soil regions.