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Effects of heavy metals on microbial biomass and activity in subtropical paddy soil contaminated by acid mine drainage

LI Yong-Tao~1, Thierry Becquer~2, Cécile Quantin~3, Marc Benedetti~4, Patrick Lavelle~2, DAI Jun~(1*) (1.College of Natural Resources and Environment, South China Agricutrual University, 510642, Guangzhou, China;2. Biodiversité et Fonctionnement des Sols, IRD,France;3. Université Paris Sud XI FRE OrsayTerre 2566 Bat. 504 91405 Cedex France;4. Laboratoire de Géochimie et Métallogénie / Université Pierre et Marie Curie, France).  
Soil samples were collected from long-term metal-contaminated rice field in Dabao Shan area of Guangdong province, China. Soil physico-chemical properties, total contents and DTPA extractable fractions of Cu, Pb, Zn and Cd, and microbial biomass and organic carbon mineralization were investigated to analyze their relationships. Principal component analysis and stepwise regression were employed to characterize availability of heavy metals, microbial responses and their effect factors in the studied soil. The results showed that irrigation with acid mine drainage since end of 1960s resulted in high sulfur content, strong acidity, relatively low organic carbon and total nitrogen, as well as high contents of heavy metals and their DTPA extractable fractions. DTPA extractable metals were not only influenced principally by their total contents, but also were positively related to soil acidity and clay contents, and were negatively correlated with soil Mn contents. Excessive heavy metals remarkably reduced soil microbial biomass carbon, biomass nitrogen, ratio of biomass carbon to organic carbon and ratio of biomass nitrogen to total nitrogen. Metals also inhibited microbial respiratory activity and rate of mineralization of soil organic carbon, finally led to increase in soil C/N. In contrast, elevated heavy metals did not significantly influence microbial biomass C/N and metabolic quotient (qCO_2), which are considered as parameters of microbial community composition and physiological metabolism. Stepwise regression indicated that DTPA extractable metals, especially DTPA-Cu, were major factors affecting microbial biomass and carbon mineralization, rather than total contents. Apart from close relationship between metals and microbial indices, some soil physico-chemical characteristics, such as organic carbon and total nitrogen, were highly associated with microbial biomass carbon, nitrogen and soil basal respiration. Some linked microbial indices including ratio of biomass carbon to organic carbon, ratio of biomass nitrogen to total nitrogen, and CO_2 evolution per unit organic carbon, can decrease the blurring effect of soil spatial variability on study of metal stress. Our results further showed that metal indices were involved in regression models of influencing factors of such linked parameters, while most of soil physico-chemical parameters were eventually excluded from the models. It demonstrates that these linked indices could serve as microbial indicators of metal stress. In addition, soil sulfur did not show pronounced effect on availability of metals and change in soil microorganism. However, oxidation of sulfide was regarded as important cause of metal ion loose and acidification. Soil physico-chemical characteristics may indirectly influence microbial biomass and carbon mineralization through their effects on availability of heavy metals.
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