Copper speciation and microbial activity in long-term contaminated soils

Most soil quality guidelines do not distinguish among the various forms of metals in soils; insoluble, nonreactive, and nonbioavailable forms are deem ed as hazardous as highly soluble, reactive, and toxic forms. The objective of this study was to better understand the long-term effects of copper on microorganisms in relation to its chemical speciation in the soil environme nt. Carbon mineralization processes and the global structure of different m icrobial communities (fungi, eubacteria, actinomycetes) are still affected after more than 50 years of copper contamination in 20 soils sampled from t wo different agricultural sites. The microbial respiration lag period (LP) preceding the beginning of mineralization process increases with the level of soil copper contamination and is not significantly affected by other env ironmental factors such as soil pH and soil organic matter (SOM) content. T he total copper concentration showed the best correlation with the LP when each site is considered separately. However, when considering the whole set of data, soil solution free Cu2+ activity (pCu(2+)) is the best predictor of Cu toxicity determined by LP (quite likely because pCu(2+) integrates th e soil physicochemical variability). The maximum mineralization rate (MMR), even if well correlated with the pCu(2+), appears not to be a good biomoni tor of copper contamination in soils since it is highly sensitive to soil c haracteristics such as SOM content. This study emphasizes the importance of the physicochemical properties of the environment on soil heavy metal toxi city and on soil toxicological measurements. These properties must be chara cterized in soil toxicological studies with respect to (1) their interactio ns with heavy metals, and (2) their direct impact on the selected biologica l test. The measurement of pCu(2+) to characterize the level of soil contam ination and of lag period as a bioindicator of metal effects in the soil ar e recognized as useful tools for the evaluation of the biological quality o f soils.