Specific enzyme activities and CHCl3-labile copper as soil indicators of phytoremediation

The aim of the study was to analyze the activity of soil microbial biomass in terms of specific enzyme activities measured in a soil historically contaminated by heavy metals in order to evaluate the effects of a phytoremediation system on soil quality restoration. The polluted area (10.000 m2) is located in Madonna dell'Acqua (Pisa, Italy). The experimental phytoremediation system, based on two tree species (2 x 2 m) (Populus nigra, var. italica L., Paulownia tomentosa (Thunb Sieb. & Zucc. ex Steud) with an interposed shrubby species (1 x 1 m) (Cystus scoparius L.), was established on February 2008. From the whole area twelve soil samples were collected at depth 0-30 cm before and after two years planting tree/shrubby species. The activity of eight enzymes, basal respiration and the CHCl3-labile copper were determined. The specific activity of soil enzymes (per unit of microbial biomass-C) and the metabolic quotient (basal respiration per unit of microbial biomass-C) were determined in order to evaluate the effect of copper bio-accumulation into microbial cells on their metabolism. The obtained results showed that the phytoremediation system was able to reduce the amount of soil copper in the polluted area. However, the DTPA-extractable copper increased with time suggesting a higher heavy metal mobility in the soil as a consequence of root exudates release. Moreover, in the two years of experiment, the CHCl3-labile copper increased, implying heavy metal accumulation into soil microbial pool. As far as microbial metabolism is concerned, the specific enzyme activities and the metabolic quotient increased with time probably because an ecophysiological stress of soil microbial biomass was established. In conclusion, the specific enzyme activities and the metabolic quotient may represent sensitive indicators to assess plant-soil-microorganisms interrelationships in heavy metals bioavailability in a phytoremediation system.