Phosphorus Fertilizer and Grazing Management Effects on Phosphorus in Runoff from Dairy Pastures
2008
Dougherty, Warwick J. | Nicholls, Paul J. | Milham, Paul J. | Havilah, Euie J. | Lawrie, Roy A.
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 P₂₀, P₄₀, and P₈₀ 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.
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