The influence of water-soluble As(III) and As(V) on dehydrogenase activity in soils affected by mine tailings

Dehydrogenase activity (DHA) in soils contaminated by arsenic-bearing tailings was correlated with total arsenic and total water-soluble arsenic (As(III)+As(V)) to evaluate the impact of tailings dispersion on the oxidative capacity of soil microorganisms. Georeferenced surface soil samples (0-10 cm depth) were collected at different distances from a tailings dam. In the samples farthest from the dam, all water-soluble arsenic (avg. 0.6±0.1 mg kg−1) was As(V). The highest concentration of water-soluble As(III)+As(V) (>1.9 mg kg−1) was found where As(III) was present. DHA averaged 438.9±79.3 μg INTF g−1 h−1 at the greatest distance from the dam and decreased to 92.3±27.1 μg INTF g−1 h−1 with decreasing distance from the dam. Pearson correlation coefficient between DHA and samples containing water-soluble As(V) (r=−0.87) was greater than that between DHA and total water-soluble arsenic (r=−0.57). The correlation between DHA and soluble arsenic containing both As(V) and As(III) was not significant (r=0.24). In soils with detectable As(III) concentrations where wet conditions prevail (i.e., reducing conditions), there is an abiotic response in addition to a biotic one. The correlation between DHA and total water-soluble As(III)+ As(V) was higher (r=−0.79) when the abiotic response was excluded. Our study demonstrated the importance of distinguishing between total and available fraction and its species and the need to evaluate biological functions in addition to purely geochemical analyses. DHA bioassay combined with other microbial properties offers a good tool for evaluating soil microbial activity and status and is a suitable indicator of the oxidative capacity of soil microorganisms affected by tailings in an oxidizing environment; however, under reducing conditions, abiotic responses must also be studied.