The vertical migration of soil nitrogen (N) losses in sloped farmlands under natural rainfall conditions remains inadequately understood. This study conducted a two-year (2023.3–2025.2) in situ runoff field monitoring experiment on purple loam slopes in Chongqing, China, systematically investigating the effects of different rainfall patterns (TR, HR, MR, LR) and planting stages (CPS, SFS, MPS, WFS) on the vertical migration of nitrogen at four depths (0, 20, 40, and 60 cm) under natural rainfall conditions. The results demonstrate that rainfall is the key driver of vertical nitrogen migration. The migration loads of total nitrogen (TN), total dissolved nitrogen (TDN), and nitrate nitrogen (NO3−-N) all increased significantly with increasing rainfall intensity (p < 0.01), showing the strongest correlation with rainfall amount in the shallow soil layer (L1). Nitrogen migration loads exhibited a clear decreasing trend with increasing soil depth, declining progressively from the surface (L1) to deeper layers (L3). However, higher loads of nitrate nitrogen were maintained in deeper layers, given its strong mobility. The study found that although extreme rainfall events (TR and HR) accounted for only 6.05% of total rainfall events, they contributed to more than 60% of the total nitrogen migration load, highlighting extreme rainfall as the primary driver of nutrient loss. Over 70% of nitrogen loss occurred during the corn planting stage (CPS) with high fertilizer demand, highlighting that this period is critical for nitrogen loss and represents a key window for risk management. The increased soil depth functions as a “sink”, exhibiting certain nitrogen retention and filtration effects. The total nitrogen content in deeper soil layers (L2, L3) shows cumulative accumulation, confirming the nitrogen migration pattern from sources (surface layers) to sinks (deep layers) within the soil profile. This study elucidates the core driving mechanisms and critical risk periods for vertical nitrogen migration in purple soil on sloped farmland, providing crucial scientific evidence for precise regional nitrogen fertilizer management and non-point source pollution control.
Wang et al. (Thu,) studied this question.