Biochar application increases biological nitrogen fixation in soybean with improved soil properties in an Ultisol

The potential improvement of biological nitrogen fixation (BNF) in legumes and edaphic factors affecting BNF remain unclear following biochar application to Ultisols. In this study, the effects of biochars derived from different feedstocks on BNF in soybean and soil properties were examined in an Ultisol. Straw biochar and manure biochar were applied at different rates (0, 24, and 48 t ha -1 ). The percentage of soybean N derived from BNF was determined by the 15 N isotope dilution method. Manure biochar and straw biochar increased BNF by 130% and 79% over the control when pooled across two application rates, respectively. However, biochar’s improvement in BNF showed a decreasing trend with the application rates. Only manure biochar application at 48 t ha -1 significantly stimulated N uptake in soybean from soil by 89% over the control. Manure biochar increased soil pH, total nitrogen, and soil available nutrients (P, K, Ca, Mg, Cu, Zn, and Mo) and reduced soil available Al to a greater extent than straw biochar. Soil pH, total nitrogen, and available nutrients (P, K, Ca, Mg, Cu, and Mo) could explain 34.5%–54.6% of the BNF variations. Among these edaphic factors, the improvement in soil available Mo induced by biochar was the most crucial to promoting BNF based on the random forest model analysis. In conclusion, biochar prompted BNF in soybean in the Ultisol, which could be attributed to the increase in soil available nutrients and the liming effect of biochar.