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

Author(s)
Herridge, David
Brock, Philippa M
Publication Date
2016
Abstract
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.
Citation
Proceedings of the 2016 International Nitrogen Initiative Conference, "Solutions to improve nitrogen use efficiency for the world", p. 1-4
Link
Publisher
International Nitrogen Initiative
Title
Annual crop legumes may not mitigate greenhouse gas emissions because of the high carbon cost of nitrogen fixation
Type of document
Conference Publication
Entity Type
Publication

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