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

Abstract

<p><i>Background</i> 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.</p> <p><i>Scope</i> 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 (<i>Glycine max</i>), 7.2 Tg for the pulses and 3.3 Tg for groundnut (<i>Arachis hypogaea</i>). 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.</p> <p><i>Conclusions</i> 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.</p>