Effects of clay addition on soil protease activities in Andosols in the presence of cadmium

Pollution by heavy metals such as cadmium (Cd) is hazardous to soil microbial function, including enzyme activities. However, the addition of clay to soils may control the negative effect of Cd by both adsorption of the metal ion onto the clay and stabilization of the soil enzymes. We investigated the effects of clay addition on the activities of soil proteases in the presence of Cd using a short-term laboratory incubation experiment. The clays were separated from the soil samples used in the study. Our experiment was done at three levels of clay and Cd addition (0, +5 and +10% of original clay content, and 0, 10 and 50 mg Cd kg-1 soil, respectively) to two surface soil samples of Andosols obtained from a forest and a cultivated field in an upland area of Japan's Kanto district. We determined the protease activities, protein content, soil pH and exchangeable Cd in both soil samples after 0, 2, 10 and 40 days of incubation. Mineralogical analysis by dissolution with acid ammonium oxalate solution on the clay fractions from both the forest and the cultivated field soil samples showed that they were largely composed of allophane (probably including imogolite) and ferrihydrite. Allophanic clay had a significant positive effect on proteases activities even in the presence of Cd, although the addition of Cd decreased protease activities, protein contents and soil pH in both soil samples in all clay treatments during incubation. Protease activities were negatively correlated with the amounts of exchangeable Cd, but the addition of clay did not have a significant recovery effect on inhibition by Cd. These results indicate that allophanic clay enhances the activity of proteases owing to stabilization of the enzyme, and that allophanic clay has little capacity to adsorb Cd2+.