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

Azospirillum brasilense and Pseudomonas fluorescens are well-known plant growth promoting rhizobacteria.However,the effects of A.brasilense and P.fluorescens 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.fluorescens contribute to the N transformations and N supply capacities in the rhizosphere,and clarified the effects of A.brasilense and P.fluorescens on the N application rate in rice cultivation.Inoculations with A.brasilense and P.fluorescens 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(M_(0)),A.brasilense(M_(b)),P.fluorescens(M_(p)),and co-inoculation with a mixture of A.brasilense and P.fluorescens(M_(bp)).The N application rate included four levels:0 kg N ha^(–1)(N_(0)),90 kg N ha^(–1)(N_(90)),180 kg N ha^(–1)(N_(180)),and 270 kg N ha^(–1)(N_(270)).The results indicated that the M_(bp) and M_(p) treatments significantly enhanced the ammonification activities in the rhizosphere compared with the M_(0) treatment,especially for higher N applications,while the Mbp and M_(b) treatments greatly enhanced the nitrogenase activities in the rhizosphere compared with the M_(0) treatments,especially for lower N applications.Azospirillum brasilense and P.fluorescens 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 N_(90),N_(180),and N_(270) 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.fluorescens.