ESTIMATES OF NITROGEN FIXATIONS BY LEGUMES IN ALTERNATE CROPPING SYSTEMS AT WARRA, QUEENSLAND, USING ENRICHED-N-15 DILUTION AND NATURAL N-15 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-N-15 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 N-15 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 N-2 fixed by the legumes. N-2 fixation measured by the enriched-N-15 dilution method in the grass-legume ley averaged 80 kg N ha(-1) year(-1) during 2 years. A similar amount of N was fixed by the lucerne ley during 1 year (83 kg N ha(-1) year(-1)), but medic ley fixed less (56 kg N ha(-1) year(-1)). The amount of N-2 fixed by chickpea was 72 kg N ha(-1) year(-1). The dry matter yield of the legumes in leys, mainly lucerne, was closely associated with the amount of N-2 fixed, with a value of 28 kg of N-2 fixed for each tonne of dry matter produced.