Nitrogen economy of annual crop and pasture legumes in southwest Australia

The proportion of legume N derived from the atmosphere by N2 fixation (%Ndfa) was assessed at a number of field sites using δ15N natural abundance methodology for the grain legumes lupin (Lupinus angustifolius L.) and field pea (Pisum sativum L.) and for subterranean clover (Trifolium subterranean L.) in annual pastures. Concurrent assessments of N accumulated in above-ground parts at peak biomass enabled N2 fixation inputs to be estimated for the two grain legumes. For lupin, %Ndfa averaged 82 (range 74–93), resulting in fixation of 177 kg N ha−1 (range 95–283) in shoot biomass. For field pea, %Ndfa averaged 74 (60–91), with the corresponding N2 fixation input in shoot biomass at 144 kg N ha−1 (range 54–165). Although showing greater fixation of N2 than field pea, lupin contained a significantly lower proportion of its total plant N in mature seed. N balance data for the two grain legumes showed that after grain removal, net fixed N input as lupin shoot residues averaged 68 kg N ha−1 and that of field pea shoot residue only 3 kg fixed N ha−1. Based on selected studies in which N in root and nodule biomass were also assessed, total potential N benefits from above and below ground residues of the two species after grain harvest were estimated to be of the order of 75 kg N ha−1 for lupin and 26 kg N ha−1 for field pea. Subterranean cloverʹs %Ndfa averaged 72 (range 0–100) in grazed pastures. Estimates of N2 fixation inputs as above-ground biomass in standing clover for a single spring sampling averaged 35 kg N ha−1 (0–82). A full seasonʹs N2 fixation input in such grazed pastures is suggested to be at least twice this value. Data are compared with N2 fixation values for these three legumes elsewhere in Australia.