Modelling symbiotic biological nitrogen fixation in grain legumes globally by LPJ-GUESS

Abstract. Biological nitrogen fixation (BNF) from grain legumes is significant importance in global agricultural ecosystems. Crops with BNF capability are expected to support the need to increase food production while reducing nitrogen (N) fertilizer input for agriculture sustainability, but quantification of N fixing rates and BNF crop yields remains inadequate. Here we incorporate two legume crops (soybean and faba bean) with BNF into a dynamic vegetation model LPJ-GUESS (Lund-Potsdam-Jena General Ecosystem Simulator). The performance of this new implementation is evaluated against observations from a range of water and N management trials. LPJ-GUESS generally captures the observed response to these management practices on legume biomass production, soil N uptake and N fixation, despite some deviations from observations in some cases. Globally, the simulated BNF is dominated by soil moisture and temperature, as well as amounts of N fertilizer addition. Annual inputs through BNF are modelled to be 11.6±2.2 Tg N for soybean and 5.6±1.0 Tg N for all pulses, with a total fixation of 17.2±2.9 Tg N yr-1 for all grain legumes during the period 1981–2016 on global scale. Our estimates show a good agreement with some previous statistical estimates but are relatively high compared to some estimates for pulses. This study highlights the importance of accounting for legume N fixation process when modelling C-N interactions in agricultural ecosystems, particularly when it comes to account for the combined effects of climate and land-use change on global terrestrial N cycle.