Nitrogen fixation, recycling and leaching

Summary
The aim of this part of the project was to estimate the spatial variability in nitrogen fixation under the influence of cattle grazing in a mixture of perennial ryegrass and white clover. Further, the 15N isotope dilution (ID) method, which implies enrichment with 15N, and the natural 15N abundance (NA) method were evaluated in reflecting the spatial variability in N2 fixation. Experiments were carried out in a grass-clover mixture at Research Centre Foulum, where the ID-method was applied, and at three farmers’ fields, where the NA-method was applied. As a result of differences in the climatic conditions between the years of investigation, a considerable variation in plant production rates and proportion of clover was registered. Consequently, the total N2 fixation in the mowed refer-ence plots was significantly lower in 1998 than in 1997, being 5.9 and 12.5 g N m-2, respectively. The plant pro-duction in these reference plots was not different from the production rates in the grazed plots. The coefficient of variance of pNdfa expressing the variability was larger at the natural abundance sites (11-40%) than at the en-riched area (5-15%). For the concentration of inorganic N the coefficient of variance ranged from 29% to 322%, and tended to increase during the grazing season. At the enriched site the correlation (r2 value) between pNdfa and inorganic N or total N in grass was 0.32, 0.12 and 0.21 for NO3--N, NH4+-N and grass-N, respectively. For the natural abundance sites the corresponding r2 values were 0.09, 0.13 and 0.001 (not significant). The total reduction in N2 fixation as a result of excretal deposits was estimated to approximately 6%.