Legume and opportunity cropping systems in central Queensland. 2. Effect of legumes on following crops

Poor yields and low grain protein in cereal crops resulting from declining soil fertility, especially nitrogen (N), are major threats to the grains industry in central Queensland. The effect of 4 different pasture-ley legumes [siratro (Macroptilium atropurpureum cv. Siratro), lucerne (Medicago sativa cv. Trifecta), lablab (Lablab purpureus cv. Highworth), and desmanthus (Desmanthus virgatus cv. Marc)] on grain yield and quality of sorghum crops was compared with that of a pulse (mungbean; Vigna radiata cv. Satin) or continuous cropping with grain sorghum (Sorghum bicolor). Legume leys consistently resulted in large increases in grain yield (188-272%), N uptake by sorghum (145-345%), and grain protein (0.21-7.0% increase in grain protein) in sorghum test-crops compared with continuous sorghum crops to which no fertiliser N had been added. The positive effect of legumes persisted up to 3 sorghum test-crops after only 1 year of legumes, although by the third year the effect was comparatively small. Mungbean and lablab generally produced the largest benefit in sorghum test-crops in the first year after legumes, whereas desmanthus and lucerne produced the least benefit. Adding fertiliser N (up to 75 kg N/ha) significantly improved grain yields and N uptake of sorghum test-crops in 3 of 4 years. However, responses to fertilisers were less than those resulting from legumes, which was ascribed to increased availability of legume N to sorghum. Legumes progressively increased soil nitrate in all subsequent sorghum test-crops (compared with continuous sorghum crops), rising from 6.8-18.9 kg NO3-N/ha after 1 year of legumes to 24.2-59.6 kg NO3-N/ha after 3 years of legumes. There was little difference between the legumes in their ability to increase soil nitrate, with the exception of desmanthus, which consistently resulted in the lowest amount of soil nitrate for subsequent test-crops and lowest uptake of N by these crops. Plant-available water content (PAWC) at planting of the sorghum test-crop was only significantly (P < 0.05) affected by previous species in 1997, when it was lowest in plots previously sown to siratro and lucerne and highest in sorghum and mungbean plots. In both 1996 and 1997, plots sown to sorghum had significantly higher PAWC at anthesis and grain maturity when previous plots were sorghum rather than legumes.