Calorimetric Analysis of Effects of a Soil-Stress Compound on Soil-Microbial Activity

Abstract

Stressing effects of the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) on soil-microbial activity were analyzed by a calorimetric method. Thirty-six mg glucose was added to 10 g of upland, orchard, paddy, sand-dune, natural-forest, and cedar-forest soils, respectively, in the presence of various concentrations of 2,4-D, and the evolved heat due to the glucose degradation was recorded at 25 ºC. The heat-evolution curves were analyzed on the basis of Richards growth model to evaluate several parameters that characterize the process of glucose degradation. The apparent degree of resistance of the soil microbes to 2,4-D was estimated from the 50 % stressing concentration K_i and the minimum inhibitory concentration MIC, which were evaluated by analyzing the 2,4-D-concentration-dependence of the degradation parameters. It was shown that the degree of resistance of the two kinds of forest soils were of an order of magnitude greater than that of the sand-dune soil in terms of the value of K_i. These stressing parameters, K_i and MIC, are valid measures for quantitative estimation of the stressing effects of pesticides on soil microbial activity.