Soil enzyme activity can decodify controversial trace elements toxicity data.

Polluted soils are systems of great complexity where toxicity due to to trace elements at high concentrations can be counteracted by a high amount of organic matter added symultaneously. Arylsulfatase (arys) beta-glucosidase (b-gluc), esterase (est), leucine aminopeptidase (leu) and alkaline phosphatase (alkP) activity were measured by microplates and fluorogenic substrates in soil extracts from 27 soil samples coming from a site illegally dumped. Soil samples were characterized for organic matter, total P and N and for both aqua regia and pore-water (rhizon soil moisture samplers) content of As, Cd, Cu, Cr, Pb and Zn, soil organic matter, total P; all these parameters were increased by the pollution event except As, which remained at the natural content, i.e. about 25 mg kg-1. The higher the pollution, the higher the content of Cu and Cd in soil solution, which were significantly correlated with soil organic matter. At the same time, however, there was an increase of enzymatic activity, clearly indicating that the beneficial effect of added soil organic matter overcame the toxic effect of trace metals on soil micro-organisms. Pore-water As was significantly correlated with total P, indicating that pollution event increased toxicity of this highly toxic trace element. According to PCA and CANCORR analysis, enzyme activities were positively correlated to As in solution, organic matter and total phosphorus. Thus, paradoxically, an increase of soluble As was paralleled by an increase of in soil enzyme activity. We hypothesize that the large amount of P added with illegal dumping competed for adsorbing sites on soil surfaces thus increasing As in solutions.