Increased ammonification,nitrogenase, soil respiration and microbial biomass N in the rhizosphere of rice plants inoculated with rhizobacteria

Azospirillum brasilense and Pseudomonas fluoresceins are well-known plant growth promoting rhizobacteria. However, the effects of A. brasilense and P. fluoresceins on the N cycles in the paddy field and rice plant growth are little known. This study investigated whether and how A. brasilense and P. fluoresceins contribute to the N transformations and N supply capacities in the rhizosphere, and clarified the effects of A. brasilense and P. fluoresceins on the N application rate in rice cultivation. Inoculations with A. brasilense and P. fluoresceins coupled with N application rate trials were conducted in the paddy field in 2016 and 2017. The inoculations of rice seedlings included four treatments: sterile saline solution (M₀), A. brasilense (M_b), P. fluoresceins (M_p), and co-inoculation with a mixture of A. brasilense and P. fluoresceins (M_bp). The N application rate included four levels: 0 kg N ha^–1 (N₀), 90 kg N ha^–1 (N₉₀), 180 kg N ha^–1 (N₁₈₀), and 270 kg N ha^–1 (N₂₇₀). The results indicated that the M_bp and M_p treatments significantly enhanced the ammonification activities in the rhizosphere compared with the M₀ treatment, especially for higher N applications, while the M_bp and M_b treatments greatly enhanced the nitrogenase activities in the rhizosphere compared with the M₀ treatments, especially for lower N applications. Azospirillum brasilense and P. fluoresceins did not participate in the nitrification processes or the denitrification processes in the soil. The soil respiration rate and microbial biomass N were greatly affected by the interactions between the rhizobacteria inoculations and the N fertilizer applications. In the M_bp treatment, N supply capacities and rice grain yields showed no significant differences among the N₉₀, N₁₈₀, and N₂₇₀ applications. The N application rate in the study region can be reduced to 90 kg N ha^–1 for rice seedlings co-inoculated with a mixture of A. brasilense and P. fluoresceins.