Biochar and Flooding Increase and Change the Diazotroph Communities in Tropical Paddy Fields

Biological nitrogen fixation (BNF) can reduce synthetic N fertilizer application and improve N-use efficiency. However, knowledge about the effect of biochar and water management regimes on soil diazotrophic microorganisms in tropical paddy fields remains only rudimentary. A field trial was started in the early rice season in 2019 and ended in the late rice season in 2020. We studied the effects of five treatments comprising different water management and biochar applications on the diazotrophic abundance and community composition: no N fertilizer + conventional water management, conventional fertilization + conventional water management, no N fertilizer + flooding, conventional fertilization + flooding, and application of 40 t ha−1 biochar + conventional fertilization + conventional water management. According to the results, biochar increased soil pH and organic carbon (SOC), whereas flooding decreased the soil available phosphorus (P) content. However, the addition of biochar and flooding as well as N application treatments increased nifH abundance. The nifH abundance negatively correlated with available N and P, whereas it significantly positively correlated with SOC (p < 0.05). The results of redundancy analysis unveiled that biochar stimulated the relative abundance of Pelomonas and changed the diazotrophic microbial community structure by increasing soil pH, while flooding stimulated the relative abundance of Azospirllum. Conclusively, both flooding and biochar affect soil diazotrophic microbial community and abundance in paddy fields. Reducing N and P fertilizer application clubbed with biochar amendment and flooding may be beneficial for soil N-fixing in tropical paddy fields.