Microbial indicators of soil health as tools for ecological risk assessment of a metal contaminated site in Brazil

Abstract

Microbial and biochemical indicators of soil health were used to assess the ecological conditions and biological activity of soils contaminated with metals at a lead smelter plant and surrounding area in northeast Brazil. Soil respiration, microbial biomass of C and N, acid phosphatase, asparaginase, and density of ammonifying and ammonium-oxidizing microorganisms were positively correlated with soil organic carbon and/or water content, but showed negative correlations with metal contents in soil. Nitrification rate and metabolic quotient (qCO2) were positively correlated with metal contamination, suggesting favorable conditions for N loss and microbial stress, respectively. No significant correlations were found between metal concentrations in soil and dehydrogenase activity or ammonification rate, considering water content and soil organic carbon as covariables. Soil respiration, microbial biomasses of C and N, dehydrogenase, acid phosphatase, asparaginase activities, and ammonifying microorganisms were positively correlated with percentage vegetation cover, while nitrification and ammonification rates were negatively correlated with this parameter. In general, soil respiration, microbial biomass of C and N, acid phosphatase, asparaginase, density of ammonifying and ammonium oxidizing microorganisms, nitrification rate and qCO2 indicated high ecological risk for soil functions mediated by microorganisms (concerning to C and nutrient cycling) due to deposition of tailing contaminated with metals, even 17 years after the smelter activities had stopped. Besides direct effect of metal toxicity on microbial biomass and activity, there are indirect effects related to changes in vegetation cover, soil organic carbon, pH, and nutrient availability, and consequently changes in the soil microclimate and physical–chemical properties that may lead to losses of habitat function for soil microorganisms and the key processes they play. However, a multivariate decomposition of variance indicated that vegetation cover explained only 3.1%, whereas metals explained 26.9% of the variation associated to the microbial/biochemical indicators, showing a stronger effect of metals.