Field evaluation of 15N techniques for estimating nitrogen fixation in legume-grass associations

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Abstract
Two techniques were used to estimate the percentage (P) of legume nitrogen fixed from atmospheric N, by subterranean clover or lucerne grown in association with either annual ryegrass or phalaris. With the 15N-enrichment technique, estimates of P for both legumes were higher when they were grown with ryegrass than with phalaris, and were lower for clover than for lucerne. The other technique, involving differences in the natural abundance of 15N, gave P values similar to those obtained by the 15N-enrichment technique, except where clover was grown with phalaris. Within 15N-enriched plots, when weed species were also used as reference plants, P was higher than when either grass species was used as the reference plant. Some aspects of the 15N-enrichment technique were examined further in the subterranean clover-annual ryegrass association. Differences in the timing and method of application of K15NO3, had no lasting effect on the estimate of P for clover. However, there was an initial increase in the P value where the K15NO3 was washed into the soil with 10 mm of water instead of 2 mm. There was no detectable nitrogenase activity associated directly with ryegrass and no fixed N2 was transferred from clover to ryegrass during the 36-day measurement period, indicating that the soil (or soil plus added 15NO-3) was the sole nitrogen source for ryegrass.