Legume and opportunity cropping systems in central Queensland. 1. Legume growth, nitrogen fixation, and water use

An experiment, established on a cracking clay (Vertisol) at Emerald, centra l Queensland, studied the dry matter (DM) production, nitrogen (N) fixation , and water use of several potential ley-legume species over 4 seasons (199 4-1997). Four ley legumes (siratro, Macroptilium atropurpureum cv. Siratro; lucerne, Medicago sativa cv. Trifecta; lablab, Lablab purpureus cv. Highwo rth; and desmanthus, Desmanthus virgatus cv. Marc) were compared with a pul se (mungbean, Vigna radiata cv. Satin), and grain sorghum (Sorghum bicolor) was included as a non-legume control. Overall, the annual legumes lablab (17.5 t/ha) and mungbean (13.4 t/ha) and the perennial siratro (16.2 t/ha) accumulated more DM than the perennials lucerne (9.6 t/ha) and desmanthus (7.1 t/ha). Lucerne produced little DM in its first year, but in later years had similar production to siratro and l ablab. Desmanthus produced >4 t/ha of DM in the first year but barely survi ved during later seasons. Annual legumes grew faster and exhausted soil water more rapidly than the p erennials. The perennials were able to extract more water from the soil tha n the annual legumes and sorghum, but were inefficient at converting small to moderate rainfall events (25-50 mm) into DM production. During the fallo w following the growth of lablab and mungbean, nitrate-N in soil increased and was always greater at the time of re-sowing than for the perennial legu mes and sorghum. Initially, the 2 annual legumes derived a high proportion (50% to >70%) of their above-ground N from fixation (%Ndfa) but this declined as the experim ent progressed to low values (<13%) in the third and fourth years, reflecti ng increased supply of nitrate from the soil. In contrast, %Ndfa peaked at 72% for siratro and >90% for lucerne, and remained high (25-50%) throughout the experiment. N fixation rates were strongly negatively correlated with soil nitrate. Over the 4 years, siratro fixed 161 kg N/ha, lucerne 120, lab lab 119, mungbean 78, and desmanthus 19 based on above-ground biomass. Mung bean had a net negative N balance (-80 kg N/ha) due to N exported in grain.