The Intergovernmental Panel on Climate Change (IPCC) standard methodology to conduct national inventories of soil N2O emissions is based on default (or Tier I) emission factors for various sources. The objective of our study was to summarize recent N2O flux data from agricultural legume crops to assess the emission factor associated with rhizobial nitrogen fixation. Average N2O emissions from legumes are 1.0 kg Nha-1 for annual crops, 1.8 kg N ha-1 for pure forage crops and 0.4 kg N ha-1 for grass legume mixes. These values are only slightly greater than background emissions from agricultural crops and are much lower that those predicted using 1996 IPCC methodology. These field flux measurements and other process-level studies offer little support for the use of an emission factor for biological N fixation (BNF) by legume crops equal to that for fertiliser N. We conclude that much of the increase in soil N2O emissions in legume crops may be attributable to the N release from root exudates during the growing season and from decomposition of crop residues after harvest, rather than from BNF per se. Consequently, we propose that the biological fixation process itself be removed from the IPCC N2O inventory methodology, and that N2O emissions induced by the growth of legume crops be estimated solely as a function of crop residue decomposition using an estimate of above- and below-ground residue inputs, modified as necessary to reflect recent findings on N allocation.
