The development of agriculture in the red soil sloping uplands has been increasingly restricted by low water availability, high temperatures, and low fertilizer use efficiency. Subsurface flow has a significant influence on runoff generation, nutrient loss, and soil erosion. The rainfall-runoff process makes it easy for nutrients on the sloping land to enter water bodies through subsurface flow mainly in the liquid phase, which may lead to environmental problems such as eutrophication and groundwater pollution. Phosphorus as one of the common nutrients causing eutrophication is immobile in the soil because it is easily absorbed and fixed by soil particles. Thus, the principal pathway of phosphorus release from the soil is the surface flow. In some regions, sufficient and concentrated rainfall results in the surface-subsurface flow that enhances phosphorus migration. Recently, researchers have studied the migration patterns of red soil phosphorus through surface flow and the impact factors arising from these migrations, as well as the generation of subsurface flow and its influence on phosphorus outputs. However, there are relatively few investigations that have comprehensively considered the influence of both surface flow and subsurface flow on the migration of red soil phosphorus. In order to investigate the characteristics of phosphorus loss through runoff under natural rainfall, a large-scale field lysimeter experiment was conducted with three treatments i.e., grass cover (GC), litter mulch (LM), and bare land (BL) on a red soil slope land in southeast China. Phosphorus loss through surface flow, interflow at different soil layers (30 cm and 60 cm), and groundwater flow (at 105 cm depth) was observed under each natural precipitation event over a one-year period. The results showed that:① The concentrations of total P (TP) and dissoluble P (DP) in surface flow were slightly higher than those in interflow and groundwater flow; the concentrations of TP and DP showed a gradual downward trend with the increase in soil depth. The total amount of TP runoff loss was ordered as BL (1.61 kg·hm-2) > LM (1.33 kg·hm-2) > GC(0.82 kg·hm-2). ② Surface flow, interflow, and groundwater contributed to 57%, 6%, and 37%, respectively, of the phosphorus runoff loss on BL plot; surface runoff was the main pathway of phosphorus loss. Groundwater flow was the crucial route of phosphorus runoff loss once a vegetation cover was in place; groundwater flow contributed to more than 71% of the phosphorus runoff loss while the surface flow contributed less than 14%. ③ Particulate phosphorus was the primary pattern of phosphorus transport which accounted for 64%-97% of the total amount of phosphorus runoff loss. The effect of phosphorus loss through groundwater flow cannot be neglected on the red soil slope land. The loss load of phosphorus through runoff can be controlled by grass cover and litter mulch treatments, whereas the concentrations of phosphorus in runoff do not significantly reduce.
Keywords: interflow; natural rainfall; phosphorus loss; red soil slops; surface runoff; underground runoff.