Little is known about the changes in soil microbial phosphorus (P) cycling potential during terrestrial ecosystem management and restoration, although much research aims to enhance soil P cycling. Here, we used metagenomic sequencing to analyse 18 soil microbial communities at a P-deficient degraded mine site in southern China where ecological restoration was implemented using two soil ameliorants and eight plant species. Our results show that the relative abundances of key genes governing soil microbial P-cycling potential were higher at the restored site than at the unrestored site, indicating enhancement of soil P cycling following restoration. The gcd gene, encoding an enzyme that mediates inorganic P solubilization, was predominant across soil samples and was a major determinant of bioavailable soil P. We reconstructed 39 near-complete bacterial genomes harboring gcd, which represented diverse novel phosphate-solubilizing microbial taxa. Strong correlations were found between the relative abundance of these genomes and bioavailable soil P, suggesting their contributions to the enhancement of soil P cycling. Moreover, 84 mobile genetic elements were detected in the scaffolds containing gcd in the 39 genomes, providing evidence for the role of phage-related horizontal gene transfer in assisting soil microbes to acquire new metabolic potential related to P cycling.

Phosphate-solubilizing bacteria (PSB) have the ability to dissolve insoluble phosphate and enhance soil fertility. However, the growth and mineral phosphate solubilization of PSB could be affected by exogenous soluble phosphate and the mechanism has not been fully understood. In the present study, the growth and mineral phosphate-solubilizing characteristics of PSB strain WS-FJ9 were investigated at six levels of exogenous soluble phosphate (0, 0.5, 1, 5, 10, and 20 mM). The WS-FJ9 strain showed better growth at high levels of soluble phosphate. The phosphate-solubilizing activity of WS-FJ9 was reduced as the soluble phosphate concentration increased, as well as the production of pyruvic acid. Transcriptome profiling of WS-FJ9 at three levels of exogenous soluble phosphate (0, 5, and 20 mM) identified 446 differentially expressed genes, among which 44 genes were continuously up-regulated when soluble phosphate concentration was increased and 81 genes were continuously down-regulated. Some genes related to cell growth were continuously up-regulated, which would account for the better growth of WS-FJ9 at high levels of soluble phosphate. Genes involved in glucose metabolism, including glycerate kinase, 2-oxoglutarate dehydrogenase, and sugar ABC-type transporter, were continuously down-regulated, which indicates that metabolic channeling of glucose towards the phosphorylative pathway was negatively regulated by soluble phosphate. These findings represent an important first step in understanding the molecular mechanisms of soluble phosphate effects on the growth and mineral phosphate solubilization of PSB.

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Phosphate-solubilizing bacteria have the ability of solubilizing mineral phosphate in soil and promoting growth of plants, but the activity of phosphate solubilization is influenced by exogenous soluble phosphate. In the present study, the effects of soluble phosphate on the activity of phosphate solubilization, acidification of media, growth, and organic acid secretion of phosphate-solubilizing bacterium Pseudomonas frederiksbergensis JW-SD2 were investigated under six levels of soluble phosphate conditions. The activity of phosphate solubilization decreased with the increase of soluble phosphate concentration, accompanying with the increase of media pH. However, the growth was promoted by adding soluble phosphate. Production of gluconic, tartaric, and oxalic acids by the strain was reduced with the increase of concentration of soluble phosphate, while acetic and pyruvic acids showed a remarkable increase. Gluconic acid predominantly produced by the strain at low levels of soluble phosphate showed that this acid was the most efficient organic acid in phosphate solubilization. Pyrroloquinoline quinone-glucose dehydrogenase gene gcd (pg5SD2) was cloned from the strain, and the expressions of pg5SD2 gene were repressed gradually with the increase of concentration of soluble phosphate. The soluble phosphate regulating the transcription of the gcd gene is speculated to underlie the regulation of the secretion of gluconic acid and subsequently the regulation of the activity of phosphate solubilization. Future research needs to consider a molecular engineering strategy to reduce the sensitivity of PSB strain to soluble phosphate via modification of the regulatory mechanism of gcd gene, which could improve the scope of PSB strains' application.

Despite a rich history of theoretical and empirical work showing that increasing biodiversity results in higher ecosystem function, this research has not made a commensurate impact on the reclamation of degraded lands, where enhancing ecosystem function is of primary importance. In this study, we manipulated plant diversity on heavily degraded mine lands and showed that increasing plant diversity greatly enhanced the reclamation of these lands. We found that high‐diversity assemblages were often associated with more biomass, higher stability and less toxic foliage than low diversity treatments, although the monocultures of Miscanthus sinensis (the most productive species) performed equally well as some of the polycultures. Our results showed that species composition and richness explained most of the total variation in biomass yield of the experimental plots, indicating that both the selection and complementarity effects influenced the positive diversity effects observed in this study. Miscanthus sinensis and legumes (as a functional group) were found to be the main contributors to the selection effect. The plots with M. sinensis tended to harbour fewer soil fungal pathogens than those without it and a similar pattern was observed for the legumes, indicating a poorly known plant–soil fungal pathogen feedback for these plants. This kind of feedback appeared to play an important role also in shaping the positive plant species richness–ecosystem function relationships recorded in the degraded mine land. More importantly, we provide the first evidence that the observed plant–soil fungal pathogen feedbacks were likely mediated by chitinolytic bacteria that release anti‐fungal enzymes. Cellulose‐degrading bacteria that aid in plant decomposition and nutrient cycling also attained higher abundances in plots with higher plant diversity, suggesting the contribution of another kind of plant–soil feedback to the positive diversity effects. Synthesis and applications. Our findings reveal that highly diverse plant assemblages are better able to spur plant–soil feedbacks and that increasing plant diversity is an important strategy to enhance land reclamation efficiency after contamination. Meanwhile, our results also indicate that some plants such as Miscanthus sinensis and legumes should be preferentially used to establish diverse plant communities for rapid reclamation of degraded lands.

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Green fluorescent protein (GFP), a renowned marker protein, is typically believed to be inert in affecting the physiology of host bacteria. We analyzed the effects of GFP-tagging on the ability of an endophytic diazotroph, Paenibacillus polymyxa P2b-2R, to fix nitrogen and promote overall growth of corn plants. The growth response and the amount of nitrogen fixed by P2b-2Rgfp-inoculated plants were compared with uninoculated controls and P2b-2R-inoculated plants at three harvests. P2b-2Rgfp inoculation significantly increased the biomass of corn plants as compared to non-inoculated controls and P2b-2R-treated plants. In vitro tests revealed that strains P2b-2R and P2b-2Rgfp possess various plant-growth-promoting characteristics, namely phosphate solubilization, production of siderophores, IAA, ammonia, and enzymes like cellulase, protease, and catalase. P2b-2Rgfp-inoculated plants fixed 18% atmospheric nitrogen, significantly higher than P2b-2R-inoculated plants (15%). This difference led us to compare the expression of structural nif genes (nifH, nifD, nifK) of strains P2b-2R and P2b-2Rgfp. It was observed that expression levels of structural nif genes of strain P2b-2Rgfp were 1.5-fold higher than those of strain P2b-2R. These results indicate that GFP-tagging positively affects the efficacy of strain P2b-2R to promote plant growth and fix nitrogen, perhaps by increasing the expression levels of structural nif genes.

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