Assessment of a soil quality criterion by means of a field survey

A field survey was conducted in order to assess a soil quality criterion as a tool for the enforcement of the soil protection law of the German federal state Baden-Wurttemberg. The law is targeted at protecting the soil in its function as a habitat for soil biota. This function may be inhibited by soil contamination with heavy metals, e.g. copper. Using the abundance of three copper-susceptible endogeic earthworm species as indicator of soil impairment in this function, the critical concentration of bioavailable copper for mineral soils with pH (CaCl2) values in the range from 4.5-7.5 was derived in a field survey on 52 sites contaminated with copper fungicides in four areas in Baden-Wurttemberg. Area-specific normal abundance of the indicator species was established at reference sites. The critical concentration was defined as the bioavailable copper content where probability of the earthworm coenosis being disturbed is 0.5. NH4NO3-extractable copper was chosen to represent the bioavailable fraction. The portion of metals belonging to the bioavailable fraction depends on soil characteristics, for copper this is especially organic matter content. A critical concentration referring to the bioavailable fraction should need no correction for these soil characteristics. In fact, derivation of the critical concentration for soils with low and high organic matter content separately yielded the same value: 677 mu g/kg. A preliminary total copper content was estimated whose surpassing indicates the possibility of the critical bioavailable copper content being exceeded. Former approaches to derive critical metal concentrations trying to extrapolate from the laboratory to the field are critically discussed. (C) 1998 Elsevier Science B.V.