The effect of soil phosphorus on particulate phosphorus in land runoff

Accumulation of surplus phosphorus (P) in the soil and the resulting increased transport of P in land runoff contribute to freshwater eutrophication. The effects of increasing soil P (19-194 mg Olsen-P (OP) kg-1) on the concentrations of particulate P (PP), and sorption properties (Q(max), k and EPCo) of suspended solids (SS) in overland flow from 15 unreplicated field plots established on a dispersive arable soil were measured over three monitoring periods under natural rainfall. Concentrations of PP in plot runoff increased linearly at a rate of 2.6 mu g litre-1 per mg OP kg-1 of soil, but this rate was approximately 50% of the rate of increase in dissolved P (< 0.45 mu m). Concentrations of SS in runoff were similar across all plots and contained a greater P sorption capacity (mean + 57%) than the soil because of enrichment with fine silt and clay (0.45-20 mu m). As soil P increased, the P enrichment ratio of the SS declined exponentially, and the values of P saturation (P(sat); 15-42%) and equilibrium P concentration (EPCo; 0.7-5.5 mg litre-1) in the SS fell within narrower ranges compared with the soils (6-74% and 0.1-10 mg litre-1, respectively). When OP was < 100 mg kg-1, P(sat) and EPCo values in the SS were smaller than those in the soil and vice-versa, suggesting that eroding particles from soils with both average and high P fertility would release P on entering the local (Rosemaund) stream. Increasing soil OP from average to high P fertility increased the P content of the SS by approximately 10%, but had no significant (P > 0.05) effect on the P(sat), or EPCo, of the SS. Management options to reduce soil P status as a means of reducing P losses in land runoff and minimizing eutrophication risk may therefore have more limited effect than is currently assumed in catchment management.