Nitrogen-Driven Reorganization of Soil N:P Across an Erosion–Deposition Gradient in Black Soil Hillslopes

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.