Effects of particulate fractions on critical slope and critical rainfall intensity for runoff phosphorus from bare loessial soil

Abstract Slope and rainfall intensity are important factors affecting runoff phosphorus (P), but the dynamic mechanism of their coupled influence on P fractions in loessial bare sloping lands is not completely understood. The critical slope and critical rainfall intensity are thresholds associated with significant changes in runoff, erosion, and/or nutrient loss. In this study, 30 rainfall events (45, 60, 75, 90, 105 and 120 mm/h; 5°, 10°, 15°, 20° and 25°) were simulated to estimate the contribution of particulate P (PP) and soluble P (SP) fractions to P loss from bare loessial soils and identify the critical slope and critical rainfall intensity for different P forms and TP fluxes. The results indicate that: (i) total phosphorus (TP) and PP loss concentrations are significantly correlated to sediment yield and slope gradient, although there were close relationships between the average TP loss concentration and rainfall intensity at slopes of 20° and 25°; (ii) P in runoff from bare loessial soil consists almost entirely of the reactive PP form (96.6%), regardless of rainfall intensity and slope. The average PP fraction for different rainfall intensities increases with increasing slope, from 94.6% at a 5° slope to 98.0% at a 25° slope; and (iii) PP and TP have the same critical slope and critical rainfall intensity for bare loessial soil because of the dominance of PP in runoff P. The critical slope for the sharp increases in PP and TP was found to be in the range of 15–20° because the average losses at 20–25° increased about 1.67 times compared to 5–15°, and the critical rainfall intensity for significant increases in PP and TP was found to be in the range of 90–105 mm/h. The hydrological pathways transporting the various P fractions under storms may help to provide new insights needed to guide sloping farmland management in rainfed land agroecosystems of loess hilly regions.