Abstract: The unique topography and climate of the Changbai Mountain has developed fertile forest soil. With modern reclamation, the region has turned into a compound ecosystem with three components of forest, forest-farmland transition and farmland. In this study, soil nutrient content, magnetization rate, and particle size grading analysis were determined for the three land uses. The soil nutrients of different ecosystems were examined and analyzed using the Mantel test with focus on the similarity between forest and farmland soil nutrients. The effect of slope change on the relationship between forest soils and farmland soils under water erosion was also analyzed. The results showed that the total organic matter of forest and farmland soils is 9.70% and 4.92%, respectively. The forest soils have the highest nutrient content, while the sloping cultivated soils are lower than that of forest soils. The changes in the coefficients of the Mantel test indicate that changes in land use affect soil properties, and tillage leads to a decrease in farmland nutrient content but an increase in the proportion of clay particles. The linear fitting of the correlation coefficients to the landform by the Mantel test shows that slope dominates the migration of soil nutrients in the cultivated soils of the Changbai Mountain region. Water erosion causes nutrients washed away in forest soils, which moves with water flow to downstream sloping farmland. The correlation coefficients of soil nutrients in different ecosystems gradually increase with increasing slope, and more nutrients are drained to sloping cropland soils. In the Changbai Mountain area, the deployment of composite ecosystem sample strips and the implementation of reasonable farming systems can effectively reduce soil nutrient loss and slow down the process of soil degradation, thus promoting the sustainable development of sloping arable land and guaranteeing food security.