Effects of the Addition of Humic Substances on Soil Protease Activities in Andosols in the Presence of Cadmium

Cadmium extraction, sorption, and immobilization seem to be the effective mechanisms in detoxification of Cd-contaminated soil. Humic substances present in soils may play an important role both in controlling the negative effects of pollution with Cd and in stabilizing soil enzymes. In this context, we have compared the effects of humic substances on soil protease activities in the presence and absence of Cd in forest and cultivated field soil samples. The samples were taken from surface soils of Andosols in a single upland area of the Kanto district in Japan. Humic substances extracted from the two soils showed differences in elemental composition, the degree of condensation of aromatic groups, and the proportions of major functional groups. Compared with the control soil samples, those with added humic substances showed a significant increase in protease activities, even in the presence of Cd (10 and 50 mg Cd kg^?1 soil). However, the addition of Cd decreased the protease activities, protein contents, and soil pH in both soil samples at each of the two levels of humic substance fortification (+5% and +10%). Moreover, protease activities showed significant negative correlation with exchangeable Cd, but adding humic substances did not lead to a reduction in either sample. Thus, although the addition of humic substances increased and stabilized protease activities, it did not lead to a clear reversal of enzyme inhibition by Cd. The obtained results indicate that in both soil samples the humic substances used in this study did not have sufficient affinity to adsorb Cd, and the impact on enzyme activities depends on the difference in chemical characteristics of the added organic matter, as suggested by the difference in enhancement of protease activities between forest and cultivated field soil samples.