Nitrogen economy of annual crop and pasture legumes in southwest Australia

Abstract

The proportion of legume N derived from the atmosphere by N 2 fixation (%Ndfa) was assessed at a number of field sites using δ 15N 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 N 2 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 N 2 fixation input in shoot biomass at 144 kg N ha −1 (range 54–165). Although showing greater fixation of N 2 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 clover's %Ndfa averaged 72 (range 0–100) in grazed pastures. Estimates of N 2 fixation inputs as above-ground biomass in standing clover for a single spring sampling averaged 35 kg N ha −1 (0–82). A full season's N 2 fixation input in such grazed pastures is suggested to be at least twice this value. Data are compared with N 2 fixation values for these three legumes elsewhere in Australia.