Copper dynamics and impact on microbial communities in soils of variable organic status.

The effect of soil organic status on copper impact was investigated by means of a microcosm study carried out on a vineyard soil that had been amended with varying types of organic matter during a previous long-term field experiment. Soil microcosms were contaminated at 250 mg Cu kg(-1) and incubated for 35 days. Copper distribution and dynamics were assessed in the solid matrix by a sequential extraction procedure and in the soil solution by measuring total and free exchangeable copper concentrations. Copper bioavailability was also measured with a whole-cell biosensor. Modifications of microbial communities were assessed by means of biomass-C measurements and characterization of genetic structure using ARISA (automated-ribosomal-intergenic-spacer-analysis). The results showed that copper distribution, speciation, and bioavailability are strongly different between organically amended and nonamended soils. Surprisingly, in solution, bioavailable copper correlated with total copper but not with free copper. Similarly the observed differential copper impact on micro-organisms suggested that organic matter controlled copper toxicity. Bacterial-ARISA modifications also correlated with the estimated metal bioavailability and corresponded to the enrichment of the Actinobacteria. Contrarily, biomass-C and fungal-ARISA measurements did not relate trivially to copper speciation and bioavailability, suggesting that the specific composition of the indigenous-soil communities controls its sensitivity to this metal.