The effect of a single application of cow urine on annual N2 fixation under varying simulated grazing intensity, as measured by four 15N isotope techniques

The effects of dairy cow urine and defoliation severity on biological nitrogen fixation and pasture production of a mixed ryegrass-white clover sward were investigated over 12 months using mowing for defoliation. A single application of urine (equivalent to 746 kg N ha(-1)), was applied in late spring to plots immediately after light and moderately-severe defoliation (35 mm and 85 mm cutting heights, respectively) treatments were imposed. Estimates of percentage clover N derived from N-2 fixation (%Ndfa) were compared by labelling the soil with N-15 either by applying a low rate of N-15-labelled ammonium sulphate, immobilising N-15 in soil organic matter, adding N-15 to applied urine, or by utilising the small differences in natural abundance of N-15 in soil. Urine application increased annual grass production by 85%, but had little effect on annual clover production. However, urine caused a marked decline in %Ndfa (using an average of all N-15 methods) from 84% to a low of 22% by 108 days, with recovery to control levels taking almost a year. As a result, total N fixed (in above ground clover herbage) was reduced from 232 to 145 kg N ha(-1) yr(-1). Moderately-severe defoliation had no immediate effect on N-2 fixation, but after 108 days the %Ndfa was consistently higher than light defoliation during summer and autumn, and increased by up to 18%, coinciding with an increase in growth of weeds and summer-grass species. Annual N-2 fixation was 218 kg N ha(-1) yr(-1) under moderately-severe defoliation compared to 160 kg N ha(-1) yr(-1) under light defoliation. Estimates of %Ndfa were generally similar when N-15-labelled or immobilised N-15 were used to label soil regardless of urine and defoliation severity. The natural abundance technique gave highly variable estimates of %Ndfa (-56 to 24%) during the first 23 days after urine application but, thereafter, estimates of %Ndfa were similar to those using N-15-labelling methods. In contrast, in urine treated plots the use of N-15-labelled urine gave estimates of %Ndfa that were 20-30% below values calculated using conventional N-15-labelling during the first 161 days. These differences were probably due to differences in the rooting depth between ryegrass and white clover in conjunction with treatment differences in N-15 distribution with depth. This study shows that urine has a prolonged effect on reducing N-2 fixation in pasture. In addition, defoliation severity is a potential pasture management tool strategically enhancing N-2 fixation.