Forage and grain legumes are integral components of sustainable crop rotations and farming systems in semiarid subtropical agricultural systems of northern Australia. The sustainability of these farming systems largely depends on the contribution of legumes to the nitrogen (N) requirement of non-legume crops. The potential legume contribution to a succeeding wheat crop was measured in a Vertisol for 17 weeks under optimum moisture (–0.006 to –0.05 MPa) or alternate wet–dry moisture regime (–0.006 to –1.2 MPa) with or without plant residue additions (4 t/ha equivalent) following the legume phase in a glasshouse experiment. The N release from the residues followed first order kinetics. Both the N release rate and net N mineralised were positively correlated with the N concentration and negatively but log-linearly correlated with carbon and N ratios of tops and roots residues. The net N mineralised and N mineralisation potentials (N₀) were not significantly affected by different moisture regimes. The proportions of net N mineralised from the annual medics, chickpea and lucerne residues were 30, 52, and 64%, respectively.

The amount of nitrate N accumulated in the field during the fallow periods from 1989–98 was not related to the amount of rainfall during these periods, and most likely limited by the amount of labile or mineralisable N. Its positive correlation with N₀ values showed that even during the relatively dry fallows (≥250 mm) in this semiarid subtropical environment, soils growing legumes could mineralise enough N to make a significant N contribution to the following crop in legume–cereal rotations.