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Abstract Background A 2-year field trial was conducted to test the effect of biochar addition (0, 15, 30, and 45 t hm−2) on soil properties, nutrients, diazotrophic community diversity, abundance, and structure, and soybean growth, yield, and quality. Furthermore, we aimed to explore the responses of diazotrophs, grain yield, and quality to nine soil environmental factors. Rhizosphere soil and plant samples were collected after harvest. Results Biochar application resulted in a lower soil bulk density (γ d) but higher total organic carbon (TOC), effective phosphorus (AP) and total nitrogen (TN). Compared with untreated soil, the diversity index of diazotrophic bacteria in biochar-amended soil decreased, but the abundance of diazotrophic bacteria increased. The microbial community remained stable when a small amount of biochar was applied but changed as biochar amount increased. Furthermore, biochar reduced the proportion of unique nitrogen-fixing bacteria, but did not affect that of common nitrogen-fixing bacteria between biochar-amended and untreated soils, and increased the relative abundance of Bradyrhizobium (B9 vs. B0) and Sinorhizobium (B18 or B21 vs. B0) involved in symbiotic nitrogen fixation. The main components and content of fatty acids (except for stearic acid) and the content of protein and soybean oil remained stable under biochar application. The low biochar treatment (15 t hm−2) promoted soybean growth and yield. Redundancy analysis suggested that TN greatly influenced the diazotrophic community structure at the phylum and genus levels, and that pH, TOC, and NO3 −-N greatly influenced grain yield and quality. Conclusions Soil diazotroph environment can be improved by targeted farmland implementation based on changes in soil physicochemical properties, which would benefit biological N fixation in agricultural soils and further increase economic benefit. Graphical Abstract |
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