Impact of co-inoculation with plant-growth-promoting rhizobacteria and rhizobium on the biochemical responses of alfalfa-soil system in copper contaminated soil

The effects and regulatory mechanisms of co-inoculation of plant-growth-promoting rhizobacteria (PGPRs) and rhizobium in plant-soil systems remain unclear, despite numerous reports that PGPRs or rhizobium can alleviate metal toxicity. We used the co-inoculation of the PGPR Paenibacillus mucilaginosus and the metal-resistant rhizobium Sinorhizobium meliloti for exploring the physiological and biochemical responses of the plant-soil system in metal-contaminated soil. The co-inoculation with the PGPR and rhizobium significantly increased the nutrient (N, P, and K) contents in plant tissues and promoted plant growth in soil contaminated with copper (Cu). Stress from Cu-induced reactive oxygen species and lipid peroxidation were largely attenuated by the co-inoculation by increasing the activities of antioxidant enzymes. The contents and uptake of Cu in plant tissues increased significantly in the co-inoculation treatment compared with the uninoculated control and individual inoculation treatment. Co-inoculation with PGPR and rhizobium significantly increased soil microbial biomass, enzymatic activities, total nitrogen, available phosphorus, and soil organic matter contents compared with the uninoculated control. Interestingly, co-inoculation also affected the composition of the rhizospheric microbial community, and slightly increased rhizospheric microbial diversity. These improvements of the soil fertility and biological activity also had a beneficial impact on plant growth under Cu stress. Our results suggested that alfalfa co-inoculated with PGPR and rhizobium could increase plant growth and Cu uptake in metal-contaminated soil by alleviating plant Cu stress and improving soil biochemical properties. These results indicate that the co-application of PGPR and rhizobium can have a positive effect on the biochemical responses of alfalfa-soil systems in soil contaminated by heavy metals and can provide an efficient strategy for the phytomanagement of metal-contaminated land.