Differentiating microbial and stabilized β-glucosidase activity relative to soil quality

Previous research has shown that β-glucosidase activity can detect soil management effects and has potential as a soil quality indicator, but mechanisms for this response are not well understood. A significant amount of hydrolytic enzyme activity comes from extracellular (abiontic) activity that is bound and protected by soil colloids. This study was conducted to determine how management affects the kinetics of this enzyme (Km, substrate affinity, and Vmax, maximum reaction velocity) and its degree of stabilization on soil colloids. Soils were sampled from three sites in Oregon, with a paired comparison within each site of a native, unmanaged soil, and a matching soil under agricultural production (>50 years). Microwave radiation (MW) stress was used to denature the β-glucosidase fraction associated with viable microorganisms in these soils as an estimate of abiontic activity. Total activity and Vmax were decreased by both management and MW. The results showed that β-glucosidase activity is sensitive to soil management on a variety of soils and environments (135 vs. 190, 80 vs. 111 and 80 vs. 134 μg PNP g−1 h−1 for managed and unmanaged treatments, respectively, at the three study sites in Oregon). The evidence suggests that this sensitivity to management is not (or minimally) due to differences in isoenzymes (Km generally was unaffected) but rather due to an overall reduction in the amount of enzyme present (Vmax decreased) and that this reduction in activity is reflected more from the activity of enzymes in the stabilized fraction than that associated with viable microbial population. Although β-glucosidase activity after MW irradiation appears to be limited as a soil quality indicator, it maybe useful as research tool to separate abiontic from microbial activity ‘biomass’ β-glucosidase activity correlated with microbial biomass C (r=0.42, P<0.05) but MW irradiated, abiontic, activity did not (r=−0.20NS).