Relative sensitivity of different soil biological properties to zinc

This study investigated the responses to Zn of selected enzymatic activities, potential nitrification and microbial activity in a soil contaminated with Zn at concentrations ranging from 74 to 3490 mg kg-1. The main aims of the study were to evaluate different models for EC50 estimation and to compare the relative sensitivity to Zn of the different biological properties studied. The EC50 for each property was calculated using three different models, namely a logistic dose response model, and a full and a partial inhibition model based on Michaelis-Menten kinetics. Both total (aqua regia extractable) and labile (CaCl2 extractable) Zn content were assessed and employed for application of the models. A response to increasing Zn concentrations was observed for most of the biological properties except for fluorescein diacetate hydrolysis activity (FDA) and acid phosphatase activity. Among the three models tested, the logistic model had the widest applicability and generally gave the best results in terms of EC50 estimation and R2 fit. Among the two kinetics models tested, the partial inhibition model was applicable only in a few cases, but in these cases results are generally better than for the full inhibition model. The utilization of either total or labile Zn data did not significantly affect the outcomes of the models. The sensitivity of the biological properties to Zn was ranked according to the models as follows: nitrate reductase > potential nitrification > β-galactosidase > dehydrogenase > phenol oxidase = urease = arylsulphatase. These ranking results were generally in accordance with evidence provided by other researchers, thus showing that the relative sensitivity of these properties, especially the most sensitive ones, is generally constant among different soil types. The relevance of this finding for soil ecological risk assessment is discussed.