Biological nitrogen fixation (BNF) is a well-known synergistic mechanism between specific bacteria and plants that converts atmospheric nitrogen (N equivalent to N) to plant-available forms. Biochar, a soil amendment, can significantly affect symbiotic activities and legume growth. This meta-analysis was conducted to evaluate the quantitative responses of various legume plants to biochar application under various soils, biochars, and climatic conditions. The data from 96 manuscripts were analyzed with regard to biomass, number of nodules, nodule weight, and BNF values of different legumes. We identified improvements in nodulation, BNF, and biomass production in response to biochar application in cool-season and forage legumes, compared to grain and warm-season legumes. Biochar enhancement of BNF and plant biomass was much higher in sandy and loamy soils in comparison to clay soils. The addition of biochar to soil with low N content, carbon (C), and cation exchange capacity (CEC) increased nodule formation, BNF, and plant biomass, compared to more fertile soils. Biochar produced under low pyrolytic temperatures (<= 550 degrees C) from plant residues with high C content (> 50%), pH < 8, and CEC > 20 cmol kg(-1) had greater improvement in BNF and plant biomass in comparison with other types of biochar. The optimum biochar rate to increase BNF and plant biomass was about 10 – 30 tons ha(-1). Overall, the biochar increased the number of nodules, nodule weights, BNF, and biomass production of leguminous plants by about 13%, 12%, 28%, and 22%, respectively.
