Does Slight Soil Acidification Negatively Affect Diazotrophic Bacterial Community in Paddy Field? A Regional-Scale Study

Diazotrophic bacteria are eco-friendly alternatives for reducing chemical N fertilizer use. Yet, their responses to soil acidification from excessive fertilization in paddy ecosystems remain unclear. To elucidate this issue, we compared the structure and function of the diazotrophic bacterial community (DBC) in acidic paddy soils (pH < 6.5) with those in neutral soils (pH > 6.5) around Lake Dongting using a geographical model that minimized interference from soil parent material, altitude, and climate. The main diazotrophic bacteria in these paddy soils belong to the phyla Pseudomonadota and Cyanobacteria. Diazotrophic cyanobacteria are resistant to soil acidification. Slight soil acidification (pH 5.5 to 6.5) increased the abundance of diazotrophic bacteria but depressed their acetylene reduction rate. Seasonal temperature variation and cation exchange capacity serve as the primary positive drivers regulating the N2 fixation of DBC. Across seasons, acidic stress increased ecological drift while decreasing homogeneous selection in the DBC. Network analyses revealed that acidification strengthened network stability via modularity and connectivity but narrowed niche breadth and favored specialists. Low temperature and acidity synergistically enhanced modular structure, revealing an adaptive strategy to sustain functionality during environmental fluctuations. This study extends the current knowledge about the effects of slight soil acidification on the structure and function of diazotrophic bacteria in paddy soils, which may be useful in enhancing the application of biological N2 fixation in acidic paddy soils.