Finding a niche for soil microbial toxicity tests in ecological risk assessment

Soil microbial toxicity tests are seldom used in ecological risk assessment s or in the development of regulatory criteria in the U.S. The primary reas on is the lack of an explicit connection between these tests and assessment endpoints. Soil microorganisms have three potential roles with respect to ecological assessment endpoints: properties of microbial communities may be endpoints; microbial responses may be used to estimate effects on plant pr oduction; and microbial responses may be used as surrogates for responses o f higher organisms. Rates of microbial processes are important to ecosystem function, and thus should be valued by regulatory agencies. However, the d efinition of the microbial assessment endpoint is often an impediment to it s use in risk assessment. Decreases in rates are not always undesirable. Pr ocesses in a nutrient cycle are particularly difficult to define as endpoin ts, because what constitutes an adverse effect on a process is dependent on the rates of others. Microbial tests may be used as evidence in an assessm ent of plant production, but the dependence of plants on microbial processe s is rarely considered. As assessment endpoints are better defined in the f uture, microbial ecologists and toxicologists should be provided with more direction for developing appropriate microbial tests.