Effects of biochar and chemical fertilizer amendment on diazotrophic abundance and community structure in rhizosphere and bulk soils

Diazotrophs carry out biological nitrogen (N) fixation process that replenishes available soil N; it is unclear how soil diazotrophic communities respond to biochar and chemical fertilizer amendment in agricultural ecosystem. Herein, we studied the impacts of biochar and chemical fertilizer amendment on diazotrophic communities in rhizosphere and bulk soils using nifH gene. The field experiment included four treatments: control (CK), biochar (B), chemical NPK fertilizer (CF), and biochar + chemical fertilizer (B + CF). nifH gene abundance in rhizosphere soils ranged from 9.00 × 10⁷ to 2.57 × 10⁸ copies g⁻¹ dry soil among the different treatments, which was 1.42–2.68 times higher compared with the bulk soils ranging from 5.83 × 10⁷ to 1.19 × 10⁸ copies g⁻¹ dry soil. Single application of biochar increased the abundance of nifH gene, whereas chemical fertilizer addition significantly decreased it in the bulk and rhizosphere soils. Single biochar addition affected diazotrophic community composition in rhizosphere soil, but not in the bulk soil. However, both CF and B + CF treatments obviously changed the community structure of diazotrophs in both soils. Moreover, rhizosphere effect enhanced nifH gene abundance and significantly altered the diazotrophic community structure compared to bulk soil. Phylogenetic analysis showed that all nifH sequences were affiliated to the cyanobacteria, α-, β-, γ-, and δ- subclasses of the proteobacteria group. Soil nutrient availability rather than pH had significant impacts on diazotrophic community structure based on mantel test and redundancy analysis. Overall, biochar improves the diazotrophic abundance, while chemical fertilization negatively affects it by altering nutrient availability, and combined application of biochar and chemical fertilizer does not counteract the adverse influences of chemical fertilizer on nitrogen-fixing microorganisms.