Inoculation of Pseudomonas sp. GHD-4 and mushroom residue carrier increased the soil enzyme activities and microbial community diversity in Pb-contaminated soils

Abstract

To remedy the heavy metal polluted soils, microbe-assisted phytoextraction was employed to remove Pb from soils. The Pb contents, soil enzyme activities, and microbial community diversity in pot experiment were determined to evaluate the effectiveness of this remediation. A Pb-resistant plant growth-promoting rhizobacteria (PGPR) was isolated to assist phytoremediation of Pb-contaminated soils by Brassica juncea. The pot experiment was divided into three groups including non-inoculated control group (CG), bacterium GHD-4 (40 mL fermentation broth with 3.04 × 1011 cfu) inoculated group (BG), bacterium GHD-4 (40 mL fermentation broth with 3.04 × 1011 cfu), and carrier (20 g mushroom residue) inoculated group (BC) and conducted at Pb concentrations of 0, 100, 200, and 400 mg kg−1, respectively. After 60 days, the Pb contents in plants and soils were measured. Activities of soil enzymes including sucrase, catalase, urease, and polyphenol oxidase were also measured. Microbial community structure and diversity were analyzed by a high-throughput sequencing technique. In the pot experiment, Pb contents in soils from BG and BC were significantly lower than those from CG (P < 0.05) and this trend was opposite to that in the plant experiment. Pb (0–400 mg kg−1) inhibited the activities of sucrase and polyphenol oxidase, while GHD-4 and GHD-4 with carrier inoculation could alleviate the inhibition, and low concentrations of Pb (< 200 mg kg−1) stimulated the activities of catalase and urease. The two phyla of Actinobacteria (17.77–48.53%) and Proteobacteria (17.85–39.66%) were dominant in all groups except for Pb400 (Pb-400 mg kg−1 group) and Pb400_b (Pb-400 mg kg−1 + bacterium GHD-4 group), where Bacteroidetes and Firmicutes were dominant. High concentrations of Pb greatly affected the bacterial community diversity, but Pb400_bc (Pb-400 mg kg−1 + bacterium GHD-4 + carrier group) could relieve the negative effect. The inoculation of GHD-4 and mushroom residue carrier could reduce the Pb contents in soil, improve the soil enzyme activity, and enrich the bacterial community diversity. This process adequately illustrates that the inoculation of GHD-4 and mushroom residue carrier effectively assisted Brassica juncea to remedy Pb-polluted soils.