Integrated phytoremediation system for uranium-contaminated soils by adding a plant growth promoting bacterial mixture and mowing grass

Plant-microorganism combined bioremediation is an efficient tool to decontaminate contaminated soils, and seeking the best matching partners between plants and microorganisms that constitute the key for success in this process. The aims of the present work were to study the effects of inoculating plant growth-promoting bacterial mixtures and different mowing heights on plant development and U-uptake capacity. First, a total of 12 mixtures consisting of Bacillus mucitaginosus, Bacillus subtilis, Citrobacter Werkman and Gillen, Bacillus cereus Frankland, and Bacillus thuringiensis in different proportions were analyzed for indole-3-acetic acid (IAA), siderophores, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase produced to screen as the inocula for phytoremediation. Second, three grass species, including Lolium multiflorum Lam., Lolium perenne L., and Dactylis glomerata L., were inoculated with the best bacterial inoculant mixtures that were M${}_4$ (10% Bacillus mucitaginosus, 20% Citrobacter Werkman and Gillen, 20% Bacillus thuringiensis), M${}_{10}$ (10% Bacillus mucitaginosus, 20% Bacillus subtilis, and 10% Bacillus cereus Frankland), and M${}_{12}$ (20% Bacillus mucitaginosus, and 10% Bacillus subtilis) selected from the first step experiment. In addition, mowing was adopted to assess the effects of mild mowing height, moderate mowing height, and heavy mowing height on U-uptake and grass growth. The results demonstrated that inoculation of M${}_4$ and M${}_{12}$ significantly improved the phytoextraction capability of three grass species along with influencing plant health and plant biomass. The extraction efficiency of L. perenne to U is higher than L. multiflorum and D. glomerata in the same concentration of U-contaminated soils. Compared with the two other plants inoculated with M${}_{12}$, the remediation effect of L. perenne was the best. Furthermore, we observed that moderate mowing height greatly influenced the plant growth and achieved a significant biomass harvest, thereby resulting in higher U-uptake compared with other treatments. The present study demonstrated that inoculation of bacteria in combination with a mowing is an approach that can be exploited to improve the efficiency of phytoextraction.