Soil erosion intensifies the redistribution and loss of soil nutrients. However, how erosion-deposition processes shape the spatial patterns of soil total nitrogen to total phosphorus (TN:TP) ratio in long-term eroded landscapes remains poorly understood. In this study, we examined the spatial variability of TN, TP, and the N:P ratio and their driving factors across a 7-ha sloping black-soil cropland in northeast China. Results showed that mean topsoil concentrations of TN and TP were 1.7 and 0.7 g kg-1, respectively, and the corresponding N:P ratio averaged 5.2, which was 46.6% lower than the Chinese average. Erosion-deposition effects were strongly depth dependent. In the 20-40 cm soil layer, all three variables declined in strongly eroded zones but increased in depositional areas, whereas in the 0-20 cm layer they were lower in depositional zones than in weakly eroded zones, indicating a vertical decoupling of nutrient redistribution under prolonged erosion. Notably, variability in soil N:P was closely associated with TN, soil organic carbon, and silt content, with TN emerging as the dominant control, as reflected by its stronger correlation with N:P (p <= 0.001) and higher variability (CV = 21.7-35.8%) relative to TP. Although elevation and slope gradient both influenced N:P spatial variability, only elevation showed a significant negative correlation (p <= 0.05). These findings indicate that, compared with TP, TN is more sensitive to long-term soil erosion and deposition and dominates the spatial pattern of the N:P ratio. The enhanced role of TN may increase the risk of N limitation in eroded farmlands. This study provides insights into the mechanisms of nutrient imbalance in eroded black soil regions and offers a scientific basis for formulating targeted soil conservation and fertility enhancement measures.