Annual crop legumes may not mitigate greenhouse gas emissions because of the high carbon cost of nitrogen fixation

A large uncertainty in constructing grain cropping Life Cycle Assessments (LCAs) is the effect of a particular crop, or sequence of crops, on soil C stocks. We propose that the C cost of legume N₂ fixation, estimated to be ca. 20 kg CO₂/kg N fixed, will be expressed as reduced residue C returned to the soil and a possible net loss of soil C. Published pre-farm + on-farm greenhouse gas (GHG) emissions associated with N-fertilised wheat (60N) and canola (100N) and N₂-fixing field pea, grown in Australia's southern grains region, were combined with modelled effects of the same crops on soil C stocks. When effects of the crops on soil C were assumed to be neutral, canola had the highest emissions at 840 kg CO₂-e/ha with field pea the lowest (530 kg CO₂-e/ha). When estimated changes in soil C were included in the LCAs, canola's GHG emission were totally offset (-100 kg CO₂-e/ha), compared with a more than doubling of emissions for field pea to 1270 kg CO₂-e/ha. This is somewhat counter-intuitive to current thinking that the substitution of fertiliser N with legume fixed N is an effective strategy for GHG emissions mitigation and highlights the need for simple, accurate methodologies for determining net changes in soil C for individual crops.