Losses of nitrogen phosphorus and sediment in runoff from hill country under different fertilizer and grazing management regimes
Lambert, M.G.; Devantier, B.P.; Nes, P.; Penny, P.E.
New Zealand Journal of Agricultural Research 28(3): 371-380
Eight 0.1-1.5 ha catchments within a grazing trial in steep hill country near Woodville, New Zealand, were monitored for 19-53 months during 1975-79. Experimental treatments with 3 grazing managements [rotational grazing with sheep (RGS) or cattle (RGC), or set stocking with sheep (SSS)] and 2 fertiliser practices [low (LF) = 11 kg P ha-1 year-1 , high (HF) = 64 kg P ha-1 year-1 + lime]. Stocking rates were 27% higher on HF than LF areas. Runoff, and nitrogen (N), phosphorus (P), and sediment concentrations in runoff, were measured. Average annual rainfall and runoff values were 1247 and 555 mm, respectively. Sediment losses were greater under RGC (2740 kg ha-1 year-1) than sheep grazing (average of RGS and SSS = 1220 kg ha-1 year-1). Total N and P losses in runoff from RGC catchments (12.1 kg N and 1.5 kg P ha-1 year-1) were higher than from sheep-grazed catchments (8.7 kg N and 0.7 kg P ha-1 year-1). No significant differences were found between RGS and SSS catchments. Although concentrations of N and P in runoff water were higher for HF than LF catchments, total N and P losses were similar because HF runoff volume was about 25% lower. The proportion of total N and P in dissolved inorganic form in runoff tended to be higher for HF than LF catchments. This study showed that increases in fertiliser application and stocking rate on hill country might not increase total N and P losses, but can increase nutrient loading of runoff waters. This may result in accelerated eutrophication.