Effects of Pb and Cd on forest soil bacterial functional diversity and community structure

重金属的毒性系数（The Toxic Factor，TF）是评价重金属潜在生态风险指数（potential ecological risk index，RI）的关键参数。为了探究基于Hakanson提出的TF值是否适用于重金属对土壤微生物的生态风险评估，以TF值为5和30的铅（Pb）和镉（Cd）构建土壤微宇宙试验，构建不同的RI水平（100、200和400），通过Biolog-ECO板和高通量测序技术分析了Pb和Cd分别对细菌功能多样性及群落结构的影响。结果表明，对照处理（CK）的细菌丰度、功能多样性（Shannon指数，Simpson指数和McIntosh指数）和基因多样性（ACE和Chao1指数）均大于Pb、Cd污染的土壤，随着RI水平的升高，Pb和Cd污染土壤中细菌的丰度、功能多样性（Shannon指数和McIntosh指数）和基因多样性（Chao1指数和ACE指数）呈下降趋势。相同RI水平下，Pb污染土壤中细菌群落的丰度、平均颜色变化率（AWCD）、功能多样性指数、OTUs数和基因多样性指数均显著大于Cd污染（P<0.05）；6大类碳源利用率及主成分（PCA）分析表明，Pb污染土壤中细菌对糖类和羧酸的利用率均显著大于Cd污染（P<0.05），在不同RI水平和重金属比例下，碳源利用模式而有所不同。同一RI水平下，相对于Pb污染，Cd污染土壤中变形菌门的相对丰度较为丰富，而绿弯菌门的相对丰度稀少；Pb和Cd污染土壤中慢生根瘤菌属、鞘脂单胞菌属、链霉菌属和norank_f__Roseiflexaceae等不同属细菌相对丰度表现出差异性。上述结果表明Hakanson提出的TF值并不适用于评估重金属Pb和Cd对土壤微生物的潜在生态风险。;The toxic factor (TF) is a key parameter commonly used to evaluate the potential ecological risk index (RI) of heavy metals. It is still unclear whether it is suitable to assess the risk on soil microorganisms by heavy metals. Here, we established soil microcosms with a gradient of RI levels (100, 200 and 400) by different combinations of lead (Pb) and cadmium (Cd) concentrations based on the TF values of 5 and 30 for Pb and Cd, respectively. Biolog-ECO microplate and high-throughput sequencing technologies were used to analyze soil bacterial functional diversity and community structure, respectively. The results showed that the abundance, functional diversity (Shannon index, Simpson index and McIntosh index), and gene diversity (Chao1 index and ACE index) of bacterial community in control soil were higher than those in the three risk levels. The bacterial abundance, functional diversity index (Shannon index and McIntosh index) and gene diversity index (Chao1 index and ACE index) significantly decreased with the increasing RI levels. At the same RI level, the abundance, average well color development, functional diversity index, OTUs (Operational taxonomic units) numbers and gene diversity index (Chao1 index and ACE index) of bacterial community in Pb contaminated soil were significantly higher than those in Cd contaminated soil (P<0.05). However, the bacterial Simpson indices in Pb contaminated soil showed no significant differences with ones in Cd contaminated soil (P>0.05) at the same level. The utilization rate of carbohydrate and carboxylic acid compounds by bacteria in Pb contaminated soil were significantly higher than those in Cd contaminated soil at the same RI level. The principal component analysis (PCA) showed that the carbon source utilization efficiencies significantly correlated with the RI levels, and the efficiencies were significantly different between Pb and Cd contaminated soils at the same level. Proteobacteria, Actinobacteria, Acidobacteria and Chloroflexi were dominant in all treatments. The bacterial community composition varied under different RI levels. At the same level, we found the relative abundance of Proteobacteria were higher in Pb contaminated soils than in Cd contaminated soils, while the relative abundance of Chloroflexi were higher in Cd contaminated soil. There were also significant differences between the relative abundance of Brachyrhizobium, Sphingomonas, Streptomyces and norank_f__Roseiflexaceae, respectively, in Pb and Cd contaminated soil. Our results indicated that the TF values proposed by Hakanson should be adjusted during the process of evaluating the potential ecological risks of Pb and Cd to soil microorganisms. This paper underlied a new exploration to mediate the TF values.