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Applied nitrogen (N) recovered by fertilised wheat and by successive wheat crops in a 4-crop sequence (1987-90) was studied by applying 15N-depleted ammonium nitrate (0, 2.5, and 7.5 g/m2) to a Vertisol in the summer-dominant rainfall region of northern Australia. Recoveries of applied N by each of the 4 crops in order of cropping sequence were 60.3� 4.2, 4.4 � 2.3, 1 . 3 � 0.49, and 0- 8 � 0.56%, there being no effect of 2 tillage treatments, conventional tillage (CT) and no till (NT), on uptake of applied N. There was very low recovery of residual fertiliser N after the first wheat crop was harvested; usually

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.