Background The demand for estimates of biological nitrogen (N) fxation inputs in agricultural systems is driven by the need to efectively manage the global N cycle.
Scope We present a methodology for quantifying N2 fxation by the grain legume-rhizobia symbioses that can be used for any year or sequence of years for which area and production statistics for legume oilseed and pulse crops were available and at country-toglobal scales. The coefcients used in the templates – harvest index, N harvest index, %N shoots, %N grain and a factor to account for below-ground N – were aggregated from 224 reports containing>4,000 observations. Values for the % total crop N derived from atmospheric N2 (%Ndfa) for specifc grain legumes and regions were determined in a companion paper. The grain legumes were estimated to fx a global total of 35.5 Tg N in the year 2018 - 25.0 Tg for soybean (Glycine max), 7.2 Tg for the pulses and 3.3 Tg for groundnut (Arachis hypogaea). Soybean dominated global grain legume N2 fxation, with 38% of total N fxed associated with soybean in South and Central America and a further 22% by soybean in North America.
Conclusions The updated estimates of N2 fxation by the grain legumes inform us of a substantial and increasing role that biological N2 fxation plays in global agricultural systems. The challenge remains to reliably estimate N inputs from other N2-fxing organisms that are active in these systems.