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Abstract:The toxic effects of heavy metal (HM) contamination on plant metabolism and soil microorganisms have been emphasized recently; however, little is known about the differences in soil physical, chemical, and biological properties between bulk and rhizosphere soils contaminated with HMs in forest ecosystem. The present study was conducted to evaluate the rhizosphere effect on soil properties, enzyme activities and bacterial communities associated with Robinia pseudoacacia L. along a HM contamination gradient. Soil organic matter (SOM), available nitrogen (AN) and phosphorus (AP) contents were significantly higher in rhizosphere soil than those in bulk soil at HM contaminated sites (p < 0.05). Compared to bulk soil, activities of four soil enzymes indicative of C cycle (β-glucosidase), N cycle (protease, urease) and P cycle (alkaline phosphatase) in rhizosphere soil across all study sites increased by 47.5%, 64.1%, 52.9% and 103.8%, respectively. Quantitative PCR (qPCR) and restriction fragment length polymorphism (RFLP) were used to determine the relative abundance, composition and diversity of bacteria in both bulk and rhizosphere soils, respectively. The copy number of bacterial 16S rRNA gene in bulk soil was significantly lower than that in rhizosphere soil (p < 0.05), and it had significantly negative correlations with total/DTPA-extractable Pb concentrations (p < 0.01). Alphaproteobacteria, Gammaproteobacteria and Firmicutes were the most dominant groups of bacteria at different study sites. The bacterial diversity index of Species richness (S) and Margalef (d Ma ) were significantly higher in rhizosphere soil compared with those in bulk soil, although no difference could be found in Simpson index (D) between bulk and rhizosphere soils (p > 0.05). Redundancy analysis (RDA) results showed that soil pH, EC, SOM and total/DTPA-extractable Pb concentrations were the most important variables affecting relative abundance, composition and diversity of bacteria (p < 0.05). Our study highlights the importance of rhizosphere effect on soil nutrient content, enzyme activity, bacterial abundance and community in HM contaminated forest soils. Further study is still required to understand the specific processes in the rhizosphere to achieve a suitable rhizosphere biotechnology for restoration of degraded forest ecosystem.

Arbuscular mycorrhizal fungi (AMF) was considered as a biotechnological tool for plant stress tolerance improvement and degraded ecosystem restoration. However, the variations in the effects of AMF species and abundance on plant performance under stress condition have to be still investigated. The study was conducted to evaluate the mechanisms of five AMF species, single or mixture, on Leymus chinensis stress tolerance along a saline-alkaline gradient. The results showed that AMF enhanced plant stress tolerance by promoting plant growth, enhancing nutrient absorption, maintaining ion balance, and improving photosynthetic efficiency. Funneliformis mosseae, Rhizophagus intraradices and the mixture treatments had more beneficial effects than Diversispora versiformis and Acaulospora scrobiculata inoculations. Plant grown under high stress level exhibited more benefits from AMF symbiosis. Our study suggests that not only presence of AMF, but also the species and abundance should be considered to reveal the exact effects of AMF on plant saline-alkaline tolerance and degraded grassland restoration.

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