Evaluation of enzyme activities in long-term polluted soils with mine tailing deposits of San Luis Potosí, México

Abstract

San Luis Potosi is one of the largest metal producers; mining activity has been responsible for metal emissions for over 100 years, from several sources (deposits, tailings, effluents, and dusts) generating effects in human and ecosystem health. The objective of this study was to evaluate the effect of the concentrations of heavy metals in the soil health of four municipalities of San Luis Potosi contaminated with mine tailings, using enzyme activity as a biochemical endpoint. Four municipalities contaminated with residues of historical mining activity were analyzed (25 topsoil samples per type of site contaminated and reference). The parameters that were analyzed included pH; organic matter (OM); electrical conductivity (EC); percentage of clay, As, Cd, Cr, Cu, Hg, Pb, and Zn; and arylsulfatase (ARS), β-glucosidase (BG), urease (UR), and fluorescein diacetate hydrolysis (FDA) activities in soil. Differences among the parameters per municipality and type of site were evaluated using a factorial analysis of variance. The relationships were analyzed by Pearson’s correlation and a stepwise distance-based linear model permutation test (DistLM). Results were visualized using a distance-based redundancy analysis (dbRDA). A hazard quotient (HQ) for metals was calculated in order to estimate the effects on soil microbial processes. A concentration gradient (mg/kg) of Zn (4744.5–65,585.7), Pb (1321.0–31,932.2), As (ND-8736.7), and Cu (130.9–8475.4) was found in the contaminated sites. The HQ showed a very high hazard level for the elements detected in all contaminated sites (1.4–655.8). The pattern of enzymatic inhibition found was ARS (95.8 %), UR (90.6 %), FDA (86.9 %), and BG (76.0 %). Strong negative relationships were observed among enzymatic activities and heavy metals in the following inhibitory effect Cu > As > Zn > Pb. Metals and covariables explained from 84 to 86 % of variability in enzyme activity. EC, Cu, and As showed a strong inhibitory effect; and parameters such as OM, pH, and clay were found to have a slightly inducing effect. In this study, the heavy metal concentrations were higher than the ones obtained in other reports for this region. The HQ reveals the presence of possible risks for the health of life in the region. The decrease of enzyme activities in soil could trigger adverse changes in the flow of matter and energy in ecosystems. This study provides a field baseline that could be part of a long-term monitoring program for these locations.