Influence of Long - term Nitrogen Fertilizer and Bradyrhizobium Inoculation on Diazotrophic Communities at Soil Interface during Soybean Cultivation

Diazotrophs are crucial for sustainable agriculture by converting atmospheric N₂ into plant-absorbable nitrogen via biological nitrogen fixation. Historically, excessive chemical fertilizer application has been employed to enhance soybean yield; however, this approach poses direct and indirect threats to the sustainable and healthy development of soil organisms. Rhizobial inoculation not only offers an eco-friendly alternative to synthetic nitrogen fertilizers but also contributes to sustainable agricultural practices. Understanding how different agricultural practices affect diazotrophic communities can provide valuable insights for optimizing nitrogen management in crop production.

In the study, we employed the nifH gene as a molecular marker to assess the impact of 10 years of nitrogen fertilization and Bradyrhizobium inoculation on diazotrophic community structure. Treatments included no fertilization (CK), phosphorus plus potassium (PK), PK plus urea (PK + N), and PK plus Bradyrhizobium japonicum 5821 (PK + R). Soil samples were collected 30 cm from the plant as bulk soil and the soil adhering to the root as rhizosphere soil.

The analysis of non-metric multidimensional scaling, neutral community model and the Spearman relationship indicated that at soybeans flowering-podding stage, Bradyrhizobium inoculation increased nifH gene copies but decreased the Shannon index in both bulk and rhizosphere soils compared to nitrogen fertilization. At maturity, Bradyrhizobium inoculation reduced nifH gene copies while increasing the Shannon index in both bulk and rhizosphere soils. Bradyrhizobium inoculation lowered beta diversity in the rhizosphere during the floweringpodding but increased it in mature bulk soil. The dominant diazotrophic genera were Bacillus, Azohydromonas, and Skermanella. Bradyrhizobium inoculation enhanced Bacillus abundance during flowering-podding but reduced it while boosting Azohydromonas and Skermanella during maturity.

Overall, Bradyrhizobium inoculation decreased network complexity but increased diazotrophic dynamics compared to nitrogen fertilization. Long-term Bradyrhizobium inoculation fosters diazotrophic interactions more effectively than nitrogen fertilization.