Phosphorus Fertilizer and Grazing Management Effects on Phosphorus in Runoff from Dairy Pastures

Fertilizer phosphorus (P) and grazing‐related factors can influence runoff P concentrations from grazed pastures. To investigate these effects, we monitored the concentrations of P in surface runoff from grazed dairy pasture plots (50 × 25 m) treated with four fertilizer P rates (0, 20, 40, and 80 kg ha −1 yr −1 ) for 3.5 yr at Camden, New South Wales. Total P concentrations in runoff were high (0.86–11.13 mg L −1 ) even from the control plot (average 1.94 mg L −1 ). Phosphorus fertilizer significantly ( P < 0.001) increased runoff P concentrations (average runoff P concentrations from the P 20 , P 40 , and P 80 treatments were 2.78, 3.32, and 5.57 mg L −1 , respectively). However, the magnitude of the effect of P fertilizer varied between runoff events ( P < 0.01). Further analysis revealed the combined effects on runoff P concentration of P rate, P rate × number of applications ( P < 0.001), P rate × time since fertilizer ( P < 0.001), dung P ( P < 0.001), time since grazing ( P < 0.05), and pasture biomass ( P < 0.001). A conceptual model of the sources of P in runoff comprising three components is proposed to explain the mobilization of P in runoff and to identify strategies to reduce runoff P concentrations. Our data suggest that the principal strategy for minimizing runoff P concentrations from grazed dairy pastures should be the maintenance of soil P at or near the agronomic optimum by the use of appropriate rates of P fertilizer.