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TECHNICAL REPORTS

Surface Water Quality

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

^a New South Wales Dep. of Primary Industries, Locked Bag 4, Richmond, NSW, Australia 2753
^b Centre for Plant and Food Science, Univ. of Western Sydney, LB 1797, Penrith South DC, NSW, Australia 1797
^c New South Wales Dep. of Primary Industries, Elizabeth Macarthur Agricultural Inst., PMB 8, Camden, NSW, Australia 2570
^d New South Wales Dep. of Primary Industries, Pasture Research Unit, PO Box 63, Berry, NSW, Australia 2535

^* Corresponding author (warwick.dougherty{at}dpi.nsw.gov.au).

Received for publication January 29, 2007. 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 x 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 x number of applications (P < 0.001), P rate x 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.

Abbreviations: AN, application number • BM, pasture biomass • DgP, dung P since fertilizer • DRP, dissolved reactive P • FP, fertilizer P rate • NSW, New South Wales • PP, particulate P • TDP, total dissolved P • TP, total P • TSF, time since fertilizer application • TSG, time since grazing