Rhizobacteria inoculation benefits nutrient availability for phytostabilization in copper contaminated soil: Drivers from bacterial community structures in rhizosphere

Plant growth-promoting rhizobacteria (PGPR) and rhizobia are potentially advantageous in improving plant growth in heavy metal contaminated soils. However, only limited information is available in literature on the manner through which the co-inoculation of PGPR and rhizobia can potentially supply nutrients to benefit plant growth in heavy metal contaminated soil. Accordingly, this study investigated the effects of Paenibacillus mucilaginosus (PGPR) and Sinorhizobium meliloti (rhizobia) co-inoculation on soil nutrients, enzyme activities, and microbial biomass in copper (Cu) contaminated soil planted with alfalfa (Medicago sativa). Moreover, we assessed soil bacterial community structure using high-throughput Illumina sequencing of 16S rRNA genes. Results showed that PGPR and/or rhizobia inoculation improved alfalfa growth. In particular, we found that this co-inoculation approach decreased Cu accumulation (48.6%) in shoots compared to the control (uninoculated). Both partial least squares path modeling (PLS-PM) and the relative importance of regressors in the linear models identified that enzyme activities, microbial biomass, and microbial community structure in Cu contaminated soil were major controlling variables of soil nutrient availability. The co-inoculation treatment significantly increased soil carbon (C) and nitrogen (N) concentrations by increasing urease (55.6%), saccharase (29.5%), and β-glucosidase (31.4%) activities compared to the control. Furthermore, the rhizosphere microbial community structure in the co-inoculation treatment was mainly regulated by soil N concentrations (i.e., both total N and available N) while altering alpha diversity (α-diversity). The relative abundances of Firmicutes (including biomarkers of the Bacillus genus) and Acidobacteria were enriched in the co-inoculated treatment, which can potentially improve soil nutrient availability and subsequently benefit plant growth. These findings indicated that the co-inoculation of PGPR and rhizobia plays an important role in promoting plant growth in Cu contaminated soil. This is because this approach can increase soil nutrient availability by enhancing soil enzyme activities and regulating rhizosphere microbial community structure.