MEASUREMENT OF BIOLOGICAL DINITROGEN FIXATION IN GRASSLAND - COMPARISON OF THE ENRICHED N-15 DILUTION AND THE NATURAL N-15 ABUNDANCE METHODS AT DIFFERENT NITROGEN APPLICATION RATES AND DEFOLIATION FREQUENCIES

A plant mixture of white clover (Trifolium repens L.), red clover (Trifolium pratense L.), and ryegrass (Lolium perenne L.) was established in the spring of 1991 under a cover-crop of barley. Treatments were two levels of nitrogen (400 and 20 kg N ha(-1)) and two cutting intensities (3 and 6 cuts per season). Fixation of atmospheric derived nitrogen was estimated by two N-15 dilution methods, one based on application of N-15 to the soil, the other utilising small differences in natural abundance of N-15. Both methods showed that application of 400 kg N ha(-1) significantly reduced dinitrogen fixation, while cutting frequency had no effect. Atmospheric derived nitrogen constituted between 50 and 64% of harvested clover nitrogen in the high-N treatment, while between 73% and 96% of the harvested clover nitrogen was derived from the atmosphere in the low-N treatment. The amounts of fixed dinitrogen varied between 31-72 kg N ha(-1) and 118-161 kg N ha(-1) in the high-N and low-N treatment, respectively. The highest values for biological dinitrogen fixation were estimated by the enriched N-15 dilution method. Estimates of transfer of atmospheric derived nitrogen from clover to grass obtained by the natural N-15 abundance method were consistently higher than those obtained by the enriched N-15 dilution method. Neither mineral nitrogen application nor defoliation frequency affected transfer of atmospheric derived nitrogen from clover to grass. Isotopic fractionation of N-14 and N-15 (B value) was estimated by comparing results for nitrogen fixation obtained by the enriched N-15 dilution and the natural N-15 abundance method, respectively. B was on average +1.20, which was in agreement with a B value determined by growing white clover in a nitrogen free media.