Soil nitrogen balances in urine-affected areas under two moisture regimes in Southland

Abstract

Nitrogen (N) balances have been drawn up for a soil with impeded drainage, which had 0 and 300 kg urine-N/ha applied to soil at 2 initial moisture levels; near wilting point (dry treatment), and near field capacity (wet treatment). Accountability was poor one day after urine application, but improved thereafter. Uptake by herbage accounted for only 15% and 22% of applied N in dry and wet treatments respectively, and NH₃-N volatilisation for 36% and 17%. Fixation of NH₄-N by clay minerals accounted for 10% in both systems and is viewed as an environmentally desirable process. After 130 days, some 30% and 40% of the urine-N in dry and wet treatments respectively remained unaccounted for. Leaching is considered to have been important only during the first wetting up of subsoil in the initially dry soil. Denitrification is suggested to be the other principal mechanism of loss, matching most of the unaccounted N loss from the initially wet soil. The general pattern of results supports an established view (Ball 1979; Ball & Keeney 1981) that grazing ruminants, by aggregating excess dietary N into urine patches, cause substantial N losses from intensively-managed pastoral ecosystems.