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) fixation inputs in agricultural systems is driven by the need to effectively manage the global N cycle. Scope: We present a methodology for quantifying N₂ fixation 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-to-global scales. The coefficients 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 specific grain legumes and regions were determined in a companion paper. The grain legumes were estimated to fix 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 fixation, with 38 % of total N fixed associated with soybean in South and Central America and a further 22 % by soybean in North America. Conclusions: The updated estimates of N₂ fixation by the grain legumes inform us of a substantial and increasing role that biological N₂ fixation plays in global agricultural systems. The challenge remains to reliably estimate N inputs from other N₂-fixing organisms that are active in these systems.