Effects of Azospirillum brasilense and Pseudomonas fluorescens on nitrogen transformation and enzyme activity in the rice rhizosphere

Azospirillum brasilense (A. brasilense) and Pseudomonas fluorescens (P. fluorescens) have long been known to benefit inoculated rice plants, but their effects on nitrogen (N) transformations and enzyme activities in the rice rhizosphere are unclear. This study explored whether A. brasilense and P. fluorescens are involved in modifying N transformations, N-supplying capacity, and enzyme activities in the rice rhizosphere, and the performance of rice biomass after inoculation with A. brasilense and P. fluorescens was also evaluated. Rice inoculation was conducted under pot culture conditions in 2014 and 2015, and the experiment included four treatments: a control (CT), rice seedling roots inoculated with A. brasilense (M-b), rice seedling roots inoculated with P. fluorescens (M-p), and rice seedling roots inoculated with a mixture of A. brasilense and P. fluorescens (M-bp). The rice variety used in this trial was Zhongzheyou 1 (Indica). To explore the effects of A. brasilense and P. fluorescens strains on N transformations and enzyme activities in the rice rhizosphere, N fertilizer was not applied in this trial, while the full doses of P (150 mg P2O5 per pot) and K (320 mg K2O per pot) were applied. Inoculating the rice rhizosphere with A. brasilense and P. fluorescens greatly improved the ammonification activities in the soil, and the effects were more favorable in the M-bp treatment. However, the contribution of inoculation to the nitrification activity in the rhizosphere was poor. Compared to CT, the average mineralized N content in the M-bp, M-b, and M-p treatments was increased by 165.7, 110.2, and 88.5%, respectively. Co-inoculation with A. brasilense and P. fluorescens in the rice rhizosphere greatly increased the nitrogenase activities in the soil, and inoculating with A. brasilense alone in the rice rhizosphere also showed good results. The microbial biomass N and enzyme activities were positively related to the N transformations in the soil. Finally, rice biomass increased greatly after A. brasilense and P. fluorescens inoculation in the rhizosphere. Inoculating A. brasilense and P. fluorescens in the rice rhizosphere accelerated N transformations and improved the N-supplying capacity of the rhizosphere soil, and increased rice biomass. The most beneficial effects were observed with A. brasilense and P. fluorescens co-inoculation in the rice rhizosphere.