Mineralisation of nitrogen contained in mature subterranean clover, capeweed and annual ryegrass, and subsequent nitrogen use by wheat in dryland farming systems in southern Australia

Net nitrogen (N) mineralisation in soil and N uptake by wheat from mature shoots and roots of subterranean clover, capeweed, and annual ryegrass, and from clover burrs were assessed with N-15-labelled plant material in 2 field studies, using confined micro-plots. In the first study, shoot residues of the 3 species (150 g DM/m(2)) were placed on the soil surface, and roots of the 3 species (75 g DM/m(2)) were mixed into 0-10 cm soil. The treatments were applied in March 1991. The shoot residues were incorporated into soil in early June 1991. Net N-15 mineralisation from the clover, capeweed, and ryegrass shoots during the 8-month experimental period was estimated to be, respectively, 15%, 12%, and 12%, and for the corresponding roots was 10%, 7%, and 6%. Negligible net mineralisation of N-15 occurred during the 2.5 months that the shoot residues were on the soil surface. Crop N-15 recoveries in wheat, at maturity, in November 1991 were 9%, 7%, and 7%, respectively, of that applied in the clover, capeweed, and ryegrass shoot residues. The respective crop recoveries from the root residues were 6%, 5%, and 3%. Less than 5% of N taken up by wheat was obtained from shoot or root residues. In a second similar study, N-15-labelled subterranean clover shoots (200 g DM/m(2)) and burrs (75 g DM/m(2)) were applied in December 1992; 3% of N-15 in the clover shoots was net mineralised during the 5 months they were on the soil surface. Crop recoveries of N-15 in October 1993, at the time of wheat anthesis, from the clover shoots and burrs were, respectively, 14% and 17% of applied N-15. The results of these field studies suggest that mature shoot residues and the associated intact roots (recoverable by wet-sieving), and clover burrs, make only a small direct contribution to the N response of cereals immediately following ley pasture in southern Australia. They also indicate that, under Mediterranean climatic conditions, generally very little net N mineralisation occurs from mature shoot residues until the shoots are incorporated into soil.