Microbiological indicators as sensitive indicators in the assessment of areas contaminated by heavy metals

Context As one of the world’s largest mining spills, the Aznalcóllar pyrite mine accident in Spain in 1998 resulted in ~45 km2 of agricultural soils polluted by arsenic and heavy metals. The Guadiamar Green Corridor (GGC) helped with soil remediation but residual pollution is still detected 20 years later. Aims Several methodologies based on chemical indicators have been used to assess the recovery of these areas. However, simple microbiological indicators (e.g. microbial activity and biomass, and metabolic quotient (qCO2)) are yet to be uesed. The aim of this study was to evaluate areas contaminated by arsenic and heavy metals using microbiological indicators of soil quality. Methods We used a systematic random sampling design to collect soil samples from two soil groups with different recovery trajectories. We analysed the total and water-soluble concentrations of arsenic (As), lead (Pb), copper (Cu), and zinc (Zn), the main soil properties, and bioassays including microbial activity and biomass, and metabolic quotient or microbial stress (qCO2). Key results Twenty years after the accident, soils with As and Pb concentrations that consistently exceeded regulatory levels had altered soil microbial biomass and functioning. Although overall rates of microbial respiration were not significantly different between polluted soils, microbial biomass was lower and qCO2 was higher in the more polluted than in less polluted soils. Conclusions The metabolic quotient and microbial biomass are sensitive indicators in the monitoring over time of soil polluted by arsenic and heavy metals. Implications Microbial indicators must be considered in the assessment of potential ecotoxicity and in the evaluation of soil biological properties influencing soil recovery in the long term.