Downslope movement of nutrients in hill pasture, Taita, New Zealand

Abstract

Concentrations of N, P, organic matter (loss‐on‐ignition), and major cations in “surface runoff” from grazed and fertilised hill pasture were measured over one year. Mean concentrations of Kjeldahl N, total P, Ca, and K were 190, 150, 24, and 240 times higher respectively than those in rainfall, and often were considerably higher than those reported in streams or water extracts of normal soils. Concentrations in runoff were low in winter because of dilution, and low in spring probably because of vigorous uptake of nutrients by plants. In summer, concentrations tended to be high because quantities of excreta or dead plant material originating from the spring and early summer flush of growth remained at the pasture surface under dry conditions. Grazing increased strongly the likelihood of high concentration of nutrients in runoff and a regression model was developed to describe seasonal variation in this effect. The peak of nitrate concentration after grazing lagged about 2 weeks after the Kjeldahl N peak and probably depended on nitrification of ammonium from dung and urine. P, Ca, and Mg concentrations increased 22‐, 6‐, and 8‐fold after superphosphate application to recently grazed pasture, but insignificant or small responses occurred under long grass. Fertiliser materials apparently moved less than 3 m downslope from the fertilised area in moderately heavy rain (38 mm in 6 h). Decline in nutrient concentration in runoff after grazing was most rapid for Kjeldahl N and K (minimum mean seasonal half‐life 8–9 days); other nutrient concentrations had a half‐life of 2–6 weeks. For Kjeldahl N, Mg, Na, and K this period apparently varied with season. The potential effect of fertiliser on P concentrations declined rapidly within the first week and stimulatory effects became insignificant within 3–4 weeks. The implications are discussed in relation to land management and water quality.