Journal
PLANT AND SOIL
Volume 254, Issue 2, Pages 469-480Publisher
SPRINGER
DOI: 10.1023/A:1025516429032
Keywords
light defoliation; moderately-severe defoliation; N-2 fixation; urine patches; white clover; N-15 isotope dilution
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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.
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