The inhibitory effect of cadmium and/or mercury on soil enzyme activity, basal respiration, and microbial community structure in coal mine–affected agricultural soil

The Cd and Hg contents in soils can be elevated due to coal mining. To estimate the effects of these two heavy metals on soil enzymes and the key microbial groups, coal mine–affected agricultural soils were cultured for 30 days with Cd and/or Hg. Soil enzyme activities were measured by a colorimetric method, and microbial abundance was assessed according to real-time quantitative PCR analysis of the 16S rRNA and 18S rRNA genes. In addition, the microbial communities were analyzed by Illumina sequencing. Heavy metals inhibited soil enzyme activities. For example, both Cd and Hg decreased 25.52–34.89% of the soil catalase activity; the highest level of Hg (30 mg kg−1) decreased 76.50–89.88% of the soil urease activity and 85.60–92.92% of the soil dehydrogenase activity; and the soil acid phosphatase activity significantly decreased by 15.18–32.64% under all the levels of Cd and decreased 17.09–30.32% under the high levels of the Cd–Hg combination (> 3 mg kg−1). In addition, increased Cd levels affected bacterial number more than fungal abundance; however, addition of Hg alone decreased the bacterial number but increased the fungal abundance. Furthermore, the bacterial communities but not fungal communities were altered by heavy metals. A total of 23 highly sensitive genera and 16 highly resistant genera were identified. The sensitive genera were assigned to Actinobacteria, Acidobacteria, Candidate division WS3, Chloroflexi, Gemmatimonadetes, Proteobacteria, and Thermotogae, while the resistant genera were affiliated to Bacteroidetes and Proteobacteria. Soils containing the highest level of the combination of Cd and Hg exhibited the lowest soil enzyme activities; bacterial communities were more sensitive to heavy metal contamination than fungi.