Comparative evaluation of soil toxicity using lettuce seeds and soil ciliates

The toxicity of elutriates from 25 putatively contaminated soils was evaluated using the standard seed (Lactuca sativa) germination test and a recently developed soil ciliate (Colpoda inflata) growth test. Correspondence in the identification of presence or absence of toxicity in the 25 soil elutriates between the two tests was 60% and concordance in ranking of toxicity between the two tests was significant (p < 0.05) using Spearman's rank correlation test (r(s) = 0.3831). Stepwise multiple regression analysis of toxicity results against soil elutriate chemistry indicated that pH explained the greatest amount of variation in response in both tests, despite high concentrations of several metals in the elutriates. For the germination test, a multiple regression model that included pH and cadmium explained 58.5% of response variation, whereas over 83% of response variation in the ciliate growth test could be explained by pH, copper, and cadmium. Differences in the response of the plant and protozoan models appeared to be due primarily to differences in their sensitivity to elutriate pH. In addition to better correspondence between elutriate chemistry and ciliate response compared to the seed germination test, lower response variability associated with the measure of ciliate growth compared with seed germination provided greater statistical sensitivity for detecting toxic effects.