Nitrogen fertiliser response of wheat in lupin-wheat, subterranean clover-wheat and continuous wheat rotations

Response of wheat to 7 rates of nitrogen (N) fertiliser was compared in clover-wheat (CW), lupin-wheat (LW) and continuous wheat (WW) rotations, in 4 alternate years on a grey gravelly sand over ironstone gravel at Badgingarra, during the period 1980-87. Nitrogen fertilisers significantly increased wheat grain yields in all assessment years (1981, 1983, 1985 and 1987). There were significant (P<0.05) interactions between rotation and N fertiliser in all years except 1983, with response to N fertiliser on wheat least in the LW rotation. The apparent average increases in N available in wheat dry matter, without added N, were 10.9 kg/ha from clover and 13 kg/ha from lupins. The contributions from clover and lupins in grain N were 10 and 12.3 kg/ha respectively. Organic carbon and total N levels in the soil were similar in the LW and WW rotations but were less than in the CW rotation. The levels of organic carbon in the LW and WW rotations decreased with time. Despite the difference in soil organic carbon and total N, grain yields were similar for the CW and LW rotations in the absence of N fertiliser but were higher than in the WW rotation. It was concluded that a LW rotation (in this environment) would be as effective, at least over a 6-year period, as a CW rotation in maintaining wheat yields due to the contribution of N from readily decomposible residues from the lupin crop. However, highest yields overall where obtained when N fertiliser was added to the CW rotation.