DENITRIFICATION AND N2O EMISSIONS FROM A UK PASTURE SOIL FOLLOWING THE EARLY SPRING APPLICATION OF CATTLE SLURRY AND MINERAL FERTILIZER

Total denitrification and nitrous oxide (N2O) losses were measured fro m three contrasting dairy management systems representing good commerc ial practice (system 1), production maintained but with reduced N loss es (system 2); and nitrate leaching less than 50 mg L-1 but with reduc ed production (system 3). Measurements were made following mineral fer tiliser application and from two plot experiments where four treatment s were applied: control, NH4NO3 at 60 kg N ha(-1), cattle slurry appli ed to the surface (equivalent to 45 kg N ha(-1)), and cattle slurry in jected. Despite low soil temperatures (<6 degrees C) and low rainfall (<3 mm), total denitrification and N2O losses peaked at 56 and 16 g N ha(-1) d(-1), respectively. Total denitrification losses decreased: sy stem 1 greater than or equal to system 2 > system 3, whereas N2O losse s decreased: system 2 > system 3 > system 1. Total denitrification los ses tended to decrease with decreasing fertiliser application rate, wh ereas fertiliser application rate was not the sole determinant of the N2O loss. The system 3 field was injected with cattle slurry for 2 yr, system 2 received some slurry by injection and system 1 received slur ry to the surface. Thus, the amount, timing and method of previous cat tle slurry application was important in determining the loss following subsequent fertiliser application. For the plot experiments, total de nitrification and N2O losses decreased in the order: slurry injected > mineral fertiliser > slurry applied to the surface > control for 5 da ys following application. However, 16 and 19 days after application, N 2O losses above the control were measured from plots that had received cattle slurry. It was inferred that the application of cattle slurry to the pasture soil stimulated greater N2O production and increased lo sses over a longer time period compared with mineral fertiliser additi ons.