The effect of toxicity of heavy metals contained in tailing sands on the organic carbon metabolic activity of soil microorganisms from different land use types in the karst region

To reveal the toxicity of multiple heavy metals from Pb–Zn tailing sands on soil microorganisms relating to soil organic C metabolic activity, 33 soil samples from different karst land use types (paddy field, corn field and citrus field) in Sidi village, Yangshuo, Guangxi, Southwest China were collected. The total concentrations of Pb, Zn, Cu and Cd, and diethylene triamine pentacetate acid (DTPA)-extractable Zn, Cu and Cd in the paddy field are higher than those in the corn and citrus field, which indicates that this area should not be used as farmland anymore according to the environmental quality evaluation standards for farmland of edible agricultural products of China (HJ/T332-2006). Due to the toxicity of multiple heavy metals from Pb–Zn tailing sands on soil microorganisms, the soil bacteria, actinomyces and fungi populations, soil microbial biomass C and N, and soil microbial quotient in the paddy field is lower than that in the corn and citrus field; however, soil organic C and total nitrogen (TN) in the paddy field are significantly higher than those in the citrus and corn field ( p < 0.05). Though C 3 and C 4 plants appear in the citrus field and corn field, respectively, the data of soil microorganisms, soil microbial biomass C and N, soil microbial quotient, soil organic C and TN have no significant difference in the citrus and corn fields ( p > 0.05), which indicates that the above results have no relation to plant photosynthesis. The carbon source utilization of microbial samples using Biolog EcoPlate assay shows that the average well color development, Shannon–Wiener diversity index ( H′ and E ), Simpson index ( D ) and McIntosh index ( U ) in the paddy field are significantly lower than those in the citrus and corn fields ( p < 0.05). Our results revealed that soil microorganisms under the effect of toxic heavy metals from Pb–Zn tailing sands can decrease the mineralization rate of soil organic C, which in turn produces the accumulation of soil organic C in the severe Pb–Zn tailing sand-contaminated area.