Inoculation of plant growth-promoting rhizobacteria and rhizobia changes the protist community of alfalfa rhizosphere soil under saline-alkali environment

Inoculation of plant growth-promoting rhizobacteria (PGPR) and rhizobia to alfalfa can improve plant resistance to abiotic stresses. However, the effects of PGPR and rhizobia on the protist community in alfalfa rhizosphere soil under saline-alkali environment have not been studied. The 18S rRNA gene high-throughput and metagenomic sequencing technologies were used to reveal changes in protist communities in the rhizosphere soil of alfalfa after inoculation with PGPR and rhizobia under saline-alkali environment. Soil properties in the rhizosphere were also measured. Inoculation with PGPR and rhizobia significantly reduced the diversity and richness of the protist communities in the rhizosphere soil of alfalfa, optimized the composition of the protist communities, and made it more beneficial to plant growth, such as the relative abundance of Ciliophora was significantly increased. In addition, and the relative abundance of protist consumers was increased, and that of parasitic protists was decreased, regardless of whether they were inoculated with a single strain or co-inoculated with PGPR and rhizobia. Inoculation with PGPR and rhizobium decreased the pH and electrical conductivity of the soil and increased the content of alkali-hydrolyzed nitrogen, available potassium, available phosphorus, and organic matter in the soil. The relationship between the soil microbial communities and various metabolic pathways was also enhanced. After inoculation with PGPR and rhizobia, changes in the diversity of the rhizosphere soil protist community and optimization of its composition, and the reduction of saline-alkali properties and increase of nutrient contents in rhizosphere soil were beneficial to plant growth under saline-alkali environment. The research provides assistance for the development of beneficial protist resources in soil, as well as the subsequent application of protist to promote plant growth under abiotic stress environment.