Field evaluation of some factors affecting nitrogen fixation in pastures by 15N isotope dilution

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Abstract
15N enrichment in Lolium perenne L. and Trifolium repens L. herbage and the proportion of nitrogen in the latter derived from symbiotic N2-fixation (PN) were measured in a temperate pasture following single or repeated applications of 50 mg N/m2 as NaNO3, (NH4)2SO4 or urea containing 10, 50 or 95 atom % 15N. In a second experiment 50 mg N/m2 as NaNO3 (50 atom % 15N) was injected either onto the soil surface or 50 mm or 100 mm below the soil surface. 15N enrichment in Lolium perenne L. herbage was generally higher on plots receiving a single application of NaNO3 after the harvest preceding measurement than on equivalent plots receiving (NH4)2SO4 or urea. Estimates of P, were consistently higher on NaNO3 plots than on (NH4)2SO4 or urea plots. Significant differences in estimates of PN between NaNO3 and (NH4)2SO4 or urea plots frequently persisted to the second harvest following 15N application, but by the third or subsequent harvest differences in estimates of P, between chemicals were only occasionally significant and showed no apparent trend. With increasing depth of 15N placement the 15N enrichment in Trifolium repens L. herbage decreased, whereas that of Lolium perenne L. remained similar. PN values therefore increased with increasing depth of 15N placement.