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 (dMa) 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.
Highlights
Soil heavy metal (HM) contamination has become one of the most severe environmental issues in many parts of the world because of their toxic effects on the environment and human health [1,2,3].HMs in soil are derived from both the natural and various anthropogenic sources [4].The major anthropogenic sources of this pollution are mining, smelting, industrial activities and the application of fertilizers or pesticides [5]
The main chemical properties of soil samples collected from different study sites are shown in
The relative abundance of the clones related to each of the major groups was calculated based on the number of clone sequences, and the results showed that the bacterial community structure was highly diverse and heterogeneous in bulk and rhizosphere soils at different HM pollution levels (Figure 4)
Summary
Soil heavy metal (HM) contamination has become one of the most severe environmental issues in many parts of the world because of their toxic effects on the environment and human health [1,2,3].HMs in soil are derived from both the natural (parent material) and various anthropogenic sources [4].The major anthropogenic sources of this pollution are mining, smelting, industrial activities and the application of fertilizers or pesticides [5]. Soil heavy metal (HM) contamination has become one of the most severe environmental issues in many parts of the world because of their toxic effects on the environment and human health [1,2,3]. HMs in soil are derived from both the natural (parent material) and various anthropogenic sources [4]. HMs are more toxic, non-biodegradable in the environment and can accumulated and magnified in organisms [8]. This kind of pollutant degrades ecosystem services and threatens human health through the food chain [9]
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