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Abstract:
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The 15N natural abundance technique was appraised for its suitability to assess the dependence of pasture legumes on N2 fixation (%Ndfa). In assessing 243 farm sites, in only 29 (12 percent) was the technique found to be unsuitable due to the error being unacceptably high. In the remaining 214 sites, subclover on average, derived 72 percent of its N from N2 fixation, medics 71 percent and serradella 76 percent, a value of less than 65%Ndfa is considered 'poor'. No consistent relationship was found in s clover based pastures between %Ndfa and any of the edaphic or management factors assessed, with the exception of total shoot Al, which suggests that reactive Al in soils may inhibit N2 fixation, however this requires further work. A detailed study at M nt Barker Research Station, W.A. revealed that the dependence of subclover on N2 fixation is lower (46%Ndfa) early in the growing season when nodules are immature and soil NO3 levels are high, but increase once NO3 levels fall. This intensive study dem
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