EFFECT OF N-FERTILIZER RATE ON THE ESTIMATION OF N2 FIXATION BY ISOTOPE-DILUTION

Lupins and wheat fertilized with different rates of (NH4Cl)-N-15 and (K2SO4)-S-35 (N:S = 10: 1) were grown in the field in laterally-confined microplots, and the proportion of biologically-fixed N2 in legume tops (P(atm)) harvested 191 days after sowing was estimated by the N-15 isotope dilution technique and the A-value modification of this technique. Estimates of P(atm) decreased markedly as the rate of N applied to lupins increased from 2.5 to 20 kg N ha-1, irrespective of the rate of N applied to the reference plant, illustrating that symbiotic N2 fixation was inhibited by rates of N fertilizer commonly used to label soil. Both nitrogen and sulphur A-values for wheat and lupins decreased as rates of fertilizer application increased, and were associated with increases in efficiencies of fertilizer N and S use by the crops. Estimates of P(atm) generally increased as the rate of fertilizer N applied to the reference plant increased from 20 to 100 kg N ha-1, but the extent of the increase was governed by the rate of N applied to the legume. Thus the fundamental requirement of the A-value modification (i.e. the independence of the reference plant A-value and rate of N application) was not fulfilled, and estimates of P(atm) using A-values were higher than estimates using the classical methodology. It was concluded that the A-value modification of the N-15 isotope dilution technique has only limited potential application in estimating the contribution of N2 fixation to plant nutrition.