Vertical Phosphorus Migration Characteristics of Purple Soil Sloping Farmland Under Maize–Mustard Rotation in the Three Gorges Reservoir Area

Maize–mustard rotation dominates sloping purple farmland in the Three Gorges Reservoir Area, where nitrogen (N) and phosphorus (P) losses jointly threaten reservoir water quality. This paper presents the P component of a combined N–P study and shares the same in situ runoff-plot platform as its parallel N migration study. A two-year in situ runoff-plot campaign at the National Purple Soil Fertility and Fertilizer Efficiency Monitoring Base quantified 0–60 cm soil P migration patterns across four rainfall regimes (TR, HR, MR, LR) and four growth stages (CPS, SFS, MPS, WFS). Results showed that rainfall runoff drove P migration both horizontally and vertically. TR and HR events, which accounted for only 6.05% of rainfall events, produced 73.26% of annual surface P loss (1.076 kg ha−1 yr−1) and 58.85% of annual subsurface leaching (0.440 kg ha−1 yr−1); over 78% of the total P load occurred during the maize planting stage. Soil depth dominated transport attenuation: PP fell from 93.42% at the surface (L0) to 52.57% at 40–60 cm (L3), and the 20–60 cm subsoil produced a net reduction of 83.33% in downward P flux leached from the topsoil, lowering the risk of P migration to deeper layers. Combined with the companion N study, both elements share extreme-rainfall drivers and the maize-stage critical loss window but diverge vertically: nitrate N shows stronger deep mobility, while the subsoil buffers P as a stable sink. This coupled N–P evidence supports site-specific control of agricultural non-point source pollution.