Linking rhizospheric diazotrophs to the stimulation of soybean N₂ fixation in a Mollisol amended with maize straw

Aims: Crop residue amendment is likely to stimulate symbiotic N2 fixation, and clarifying its effect on N2-fixing bacteria, i.e., diazotrophs in the rhizosphere of legume crops, is important for sustainable N management in legume-cereal cropping systems. Therefore, this study aimed to reveal the diazotrophic community composition in the rhizosphere of soybean in response to maize residue amendment. Methods: Being designed with treatments of maize residue, chemical fertilizer, and non-fertilizer applications, this study deployed the 15N-labeling technology combined with high-throughput sequencing of the nifH gene as a molecular marker for diazotrophs to quantify the symbiotically-fixed N2 in soybean plants and link symbiotically fixed N2 to the diazotrophic community diversity in the rhizosphere. Results: Residue amendment increased the abundance of diazotrophs and fundamentally altered the composition of its community in the rhizosphere. It increased the relative abundances of Bradyrhizobium and Azohydromonas compared to the chemical fertilizer treatment. The copy number of nifH in the rhizosphere was associated with dissolved organic carbon and N2 fixation. Conclusions: Residue-induced increase in dissolved organic carbon may provide sufficient carbon sources for diazotroph enrichment and thus enhance nodulation. The maize residue amendment may enrich N2 fixers to facilitate nodulation and subsequent N2 fixation of soybean, highlighting the eco-functional importance of diazotrophs fixing extra N into the rotation system.