An evaluation of 15N methods for estimating nitrogen fixation in a subterranean clover-perennial ryegrass sward

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Abstract
Nitrogen (N2) fixation by nodulated subterranean clover, in swards with perennial ryegrass, was studied by using the natural abundance of 15N in sward components compared with a method using artificial enrichment of the soil with small amounts of K15NO3. Significant differences between the 15N concentrations in ryegrass and clover enabled yield-independent estimates of the proportion (P) of clover nitrogen fixed from atmospheric N2. Yield-dependent estimates of P were also made during intervals of growth in autumn and in spring. Values of P increased with time and during spring were close to l00%, when maximum fixation rates were approximately 4 kg N ha-1 day-1. Consistent differences in 15N concentration of shoots and roots had little effect on P. Early in the experiment, natural enrichment gave lower estimates of P than 15NO-3 -enriched treatments. Yield-independent and yield-dependent methods gave similar estimates of P. During winter, when no net growth or nitrogen accumulation was recorded, there appeared to be loss of 15N from the plants, possibly because of loss of highly labelled plant parts, balanced by slow growth of tissue containing a lower 15N concentration. During winter, calculation of P was therefore unreliable.