Nutrient losses in drainage and surface runoff from a cattle-grazed pasture in Southland

Abstract

In response to local concerns about the expanding Southland dairy herd, a 4-year study was initiated in 1995 with the primary objective of quantifying nitrate-N losses to waterways from intensively grazed cattle pastures. Treatments were annual N fertiliser inputs of 0, 100, 200 or 400 kg N/ha. Stocking rate was set according to the pasture production on each of these four treatments, and over the 4 years of study ranged between the equivalent of 2.0 cows/ha for the 0N treatment, to 3.0 cows/ha for the treatment receiving 400 kg N/ ha/year. Mean annual losses of nitrate-N in drainage were 30, 34, 46 and 56 kg N/ha for the 0, 100, 200 and 400 kg N/ha/year treatments, respectively. Corresponding mean nitrate-N concentrations in drainage waters were 8.3, 9.2, 12.5 and 15.4 mg/ l, respectively. Very little direct leaching of fertiliser N was observed, even for drainage events in early spring, shortly after urea fertiliser application. The increased nitrate-N losses at higher rates of N fertiliser addition were instead owing to the indirect effect of increasing returns of urine and dung N to pasture. In Years 2 and 3, leaching losses of Ca, Mg, K, Na and sulphate-S averaged 61, 9, 11, 28 and 17 kg/ha/year, respectively, in the 0N fertiliser treatment. Increasing fertiliser N inputs significantly increased calcium and, to a lesser extent, potassium leaching losses but had no effect on losses of other plant nutrients. Surface runoff losses of Total-P, nitrate-N and ammonium- N were less than 0.5 kg/ha/year. For this well-drained Fleming soil, surface runoff was a relatively minor contributor of N to surface water, even for plots receiving high rates of fertiliser N and at a stocking rate of 3.0 cows/ha. Extrapolating these results to a 'typical' dairy pasture in Eastern Southland would suggest that the safe upper limit for annual fertiliser N additions to this site to achieve nitrate in drainage water below the drinking water standard is approximately 170 kg N/ha. Although losses of Ca in drainage were large, returns of this nutrient in maintenance applications of superphosphate-based products and lime should ensure Ca deficiencies are avoided in Southland dairy pastures. Keywords: cation-anion balances, dairy, N fertiliser, nitrate leaching, surface runoff, Southland