No abstract

Summary. Legumes have become common rotation crops in cotton cropping systems in northern New South Wales. Levels of nitrogen fixation and yield achieved on-farm were measured in commercial faba beans and other winter and summer legume crops sown after cotton over 3 years to assess the relative inputs of fixed nitrogen (N) into this system. Faba bean crops fixed up to 350 kg N/ha, removed up to 160 kg N/ha in harvested grain and contributed up to 270 kg fixed N/ha to soil N after harvest. Grain yields, N2 fixation and dry matter production were reduced in late-sown crops and those water-stressed during pod-filling, but most faba bean crops fixed almost 3 times as much N as was removed in grain. Below-ground legume N, determined with 15N shoot feeding techniques, accounted for 40% of the total crop N at peak biomass, or about 100 kg N/ha for the average faba bean crops. Residual fixed N after harvest was predicted from crop dry matter and grain yield, and this could be used to assess the contribution to soil N from faba beans. Amounts of nitrogen fixed by other legume crops ranged from 20 kg N/ha for adzuki bean and droughted lablab to more than 450 kg N/ha by irrigated soybean. Soybean, peanut and Dolichos lablab contributed more fixed N to the soil than adzuki bean, mung bean or pigeon pea under irrigated conditions. Winter crops including field peas, lentils and lupins and green-manured pasture species fixed up to 240 kg N/ha.

The effects of different management regimes on N2 fixation by subterranean clover (Trifolium subterraneum) in annual pastures and lucerne (Medicago sativa) in perennial-based pastures were examined in 5 experiments and 55 commercial paddocks, in which the pastures were grown in phased rotation with crops. The objectives were to quantify the inputs of fixed N2 and to determine ways of increasing nitrogen (N) inputs into ley-farming systems of southern New South Wales and north-eastern Victoria. Estimates of annual amounts of N2 fixed, based on above-ground herbage production in grazed pastures, ranged from 5 to 238 kg N/ha for subterranean clover and from 47 to 167 kg N/ha for lucerne. Legume reliance upon N2 fixation for growth (Pfix) was high (>65%) in most annual and perennial pastures examined. The levels of Pfix were generally unaffected by management treatments. As a consequence the amounts of N2 fixed were predominantly regulated by the legume content and herbage yield of pastures rather than by any marked differences in the ability of the legume to fix N. When all experimental results were combined with on-farm measurements of N2 fixation, the data indicated that lucerne and subterranean clover fixed 22-25 kg N for every tonne of legume dry matter produced. Management inputs to annual pastures which improved the productivity of subterranean clover and the amounts of N2 fixed included applications of superphosphate and the removal of grass species with herbicide, although the response to these treatments was not consistent across all sites in all years. Potential inputs from N2 fixation were high in annual pastures, and improved management during a good clover season enhanced the levels of mineral N detected in the soil profile (0-200 cm) the following autumn by 100-200 kg N/ha. However, year-to-year variability in annual pasture productivity and clover content resulted in large fluctuations in amounts of N2 fixed. Perennial pastures containing lucerne provided consistently greater annual herbage production, had more stable legume contents, and fixed on average 90-150% more N2 than neighbouring subterranean clover-based pastures. Even during the 1994 drought when annual pastures failed, lucerne still managed to fix >70 kg N/ha. It is proposed that lucerne-based pastures could represent a more reliable means of improving soil fertility for subsequent crops than annual pastures.

No abstract