A recent open-access paper Iannetta et al. 2016 gives a comprehensive account of the contribution of legume crops and forages to the nitrogen cycle in temperate agriculture.
Iannetta et al. 2016. A comparative nitrogen balance and productivity analysis of legume and non-legume supported cropping systems: the potential role of biological nitrogen fixation. Frontiers in Plant Science, 21 November 2016 http://dx.doi.org/10.3389/fpls.2016.01700
Using data from existing field experiments across Europe, nitrogen fixation by legumes was estimated as a residual when other main fluxes of N were accounted for. Annual fixation ranged from 30 to 115 kg/ha of nitrogen. For comparison, the upper figure is a little higher than the fertiliser N given each year to spring cereals.
Why are these figures important? There’s a dearth of estimates or measures of biological N fixation in north temperate agriculture. But while such information is essential for designing sustainable systems, it is not in itself considered to be high-profile (and therefore fundable) science.
Hence the need to add value to existing datasets to get these first estimates. Work is in progress to measure actual fixation rate in arable fields.
Hutton Agroecology group contributions to this paper are as follows. Pete Iannetta developed the overall concept and fronted the paper. Graham Begg designed and led the statistical analysis and modelling. PI, GB and Mark Young carried out the N balance calculations. Geoff Squire and Euan James offered specialist insights.
Page 2 gives some background to the article.
Images: the red clover looked to be part of a natural patch (i.e. not sown) growing locally; the vetch root and nodules were unearthed on a field trip in Attica, Greece during the EU Legume Futures project.