Response of β-Glucosidase and Proteases to Cadmium Addition in Andosols From a Forest and a Cultivated Field

Abstract

Cadmium (Cd) is considered to be the metal with the most serious adverse effects on microbial and enzyme activities in soils polluted with heavy metals. To investigate its effects, we conducted a short-term laboratory experiment at five levels of Cd addition to the soils (0-, 5-, 50-, 100-, and 250-mg of Cd kg−1 soil) using surface soil samples from Andosols obtained from a forest and a cultivated field in an upland area of Japan's Kanto District. We determined the activities of β-glucosidase and proteases in the soils after 0, 2, 10, 20, 40, and 80 days of incubation. Availability of Cd was determined by measuring water-soluble Cd. We also measured these enzyme activities after the addition of Cd to commercial crude β-glucosidase and proteases and after the addition of these crude enzymes and Cd to autoclaved soils to evaluate the stability of the enzymes and to detect their changes in response to Cd. During the incubation experiment, Cd inhibited protease activity by up to 47% in the forest soil and 31% in the cultivated soil, where negative correlation was found between soluble Cd content and protease activity on each incubation day. In contrast, β-glucosidase activity showed little or no significant inhibition. For crude enzyme experiments, Cd did not significantly affect β-glucosidase activity, but the four proteases we tested were strongly inhibited at different levels of Cd concentrations. Thus, the responses of the β-glucosidase and protease activities of the crude enzymes to Cd addition were similar to the results for the Andosols with Cd addition. As a result, the apparent direct inhibitory effect of Cd on protease stability would result in a greater decrease in N mineralization than in cellulose decomposition by β-glucosidase.