MEASUREMENTS OF NITROGEN-FIXATION IN FABABEAN AT DIFFERENT N-FERTILIZER RATES USING THE N-15 ISOTOPE-DILUTION AND A-VALUE METHODS

The N-15 isotope dilution and A-value methods were used to measure biological nitrogen (N2) fixation in field grown fababean (Vicia faba L.), over a 2-year period. Four N rates, 20, 100, 200 and 400 kg N ha-1 were examined. The two isotope methods gave similar values of % N derived from the atmosphere (%Ndfa). With 20 kg N ha-1, %Ndfa in fababean was about 85% in both years. Increasing the N rate to 100 kg N ha-1 decreased N2 fixation slightly to 75%. Further reductions in N2 fixed to 60 and 43% occurred where 200 and 400 kg N ha-1 were applied, respectively. Thus even higher rates of N than normally applied in farming practice could not completely suppress N2 fixation in fababean. We also devised one equation for both the isotope dilution and A-value approaches, thereby (i) avoiding the need for different calculations for the N-15 isotope methods, and (ii) showing once again that the isotope dilution and A-value methods are mathematically and conceptually identical.