Estimates of nitrogen fixations by legumes in alternate cropping systems at Warra, Queensland, using enriched - 15N dilution and natural 15N abundance techniques

Nitrogen fixation was measured using two isotopic techniques over 2 years as part of a long-term field experiment established to test alternative management strategies for restoring fertility in a vertisol at Warra, Southern Queensland. Treatments containing legumes were: grass- legume ley (purple pigeon grass and Rhodes grass, lucerne and annual medics) for 4 years followed by 4 years of wheat; a 2-year rotation of lucerne and wheat; a 2-year rotation of medic and wheat; and a 2-year rotation of chickpea and wheat. For the enriched-15N procedure, the proportion of N derived from air (% Ndfa) for the grass-legume and lucerne and medic leys ranged from 67 to 97%, and averaged 85%, with little evidence for effects of season, pasture establishment, time or species. The % Ndfa for chickpea was significantly lower (62%). Values for the natural abundance 15N procedure were mostly lower and more variable than for the enriched method, ranging from 62 to 91% for the grass-legume, lucerne and medic leys, and averaged 76%. It was concluded that the enriched procedure provided more reliable estimates of N2 fixed by the legumes. N2 fixation measured by the enriched-15N dilution method in the grass-legume ley averaged 80 kg N ha-1 during 2 years. A similar amount of N was fixed by the lucerne ley during 1 year (83 kg N ha-1 but medic ley fixed less (56 kg N ha-1 The amount of N2 fixed by chickpea was 72 kg N ha-lyear-1. The dry matter yield of the legumes in leys, mainly lucerne, was closely associated with the amount of N2 fixed, with a value of 28 kg of N2 fixed for each tonne of dry matter produced.