Gaseous losses of nitrogen from grassland

Abstract

The major processes involved in gaseous losses of nitrogen (N) from grassland are denitrification and ammonia (NH₃) volatilisation. The most widely used methods to measure such losses are based on soil covers and micrometeorological techniques. The former are used primarily in measuring nitrous oxide and denitrification loss in conjunction with ¹⁵N or the acetylene-inhibition technique. In studies of NH3 loss, micrometeorological techniques are generally preferred, the mass balance method being the best suited to most glazing experiments. In systems dependent on NH₄NO₃ fertilizer, denitrification is the major pathway of direct loss from the N applied with annual losses of between 5 and 10% for cut swards. Higher rates of loss are observed in grazed swards, and are to be expected following the application of slurry. Losses of NH3 from urea fertilizer range from 5 to 20%. Depending on weather conditions, a 4 to 66% loss of N has been observed for urine- and dung-affected areas of pasture whilst losses of 20 to 120 kg N ha⁻¹ yr⁻¹ have been reported for whole grazed swards. Large losses of NH₃ have also been inferred from measurements following the application of slurry. Direct denitrification loss from NH₄NO₃ fertilizer is reduced substantially when ammoniacal fertilizers are used. Reduction of the NH₃ loss associated with excreta is to be expected with irrigation, amendment of cation exchange capacity in the surface soil and by injection of slurry. Further field studies are required to establish the principal factors affecting gaseous losses of N and to identify appropriate management through which they may be reduced.