Quantifying country-to-global scale nitrogen fixation for grain legumes II. Coefficients, templates and estimates for soybean, groundnut and pulses

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.