Modeling the effects of temperature and moisture on soil enzyme activity: Linking laboratory assays to continuous field data

Although potential enzyme activity measurements have a long history of use as an indicator of microbial activity, current methods do not provide accurate estimates of in situ activity. In the field, diffusion rates typically limit the rate at which enzymes can pair with substrates. However, the common laboratory practice of creating soil slurries removes all diffusion constraints. In addition, temperature strongly affects in situ enzyme activities, but is rarely considered in enzyme assays. To address these limitations, we developed a new protocol to measure the moisture and temperature sensitivity of enzyme activities. We incorporated sensitivity data obtained using this protocol into a model to estimate the effects of temperature and moisture on in situ beta-glucosidase enzyme activity, recognizing that other factors such as substrate concentrations and diffusion constraints also affect in situ enzyme activities. Soil samples were collected from the Boston-Area Climate Experiment every two weeks over a 10-week period to track enzyme dynamics as field temperature and moisture changed. Precipitation inputs to an old-field were manipulated to produce drought (50% ambient precipitation), ambient, and wet (150% ambient precipitation) treatments. Temperature sensitivity of beta-glucosidase was determined by assaying for the enzyme in soil slurries at three different temperatures (15, 25 and 35 degrees C). Moisture sensitivity was determined by exposing soils to different moisture levels in the lab and adding substrate to homogenized dry or moist soils instead of slurries. Temperature sensitivity was calculated as Q(10) and moisture sensitivity was calculated using a linear regression for each field treatment at each sample collection date. Moisture sensitivity varied significantly among the five sample dates and treatments, whereas temperature sensitivity remained stable. At almost every time point, beta-glucosidase activity responded more strongly to increased moisture in soils of drought plots than in soils of ambient and wet plots. We estimated in situ beta-glucosidase activity in the fall using the temperature and moisture sensitivities. Estimates that used only temperature or only moisture sensitivity suggested that ambient plots had the highest activity, followed by wet and then drought plots. Estimates based on both temperature and moisture suggested that beta-glucosidase activity responded primarily to changes in temperature, except when soils were dry, with water potentials below -1 MPa. These results demonstrate that low soil moisture can strongly limit in situ enzyme activity in soils, negating any positive effect of warming. This study provides a template for parsing out the role of specific abiotic drivers on in situ enzyme activities, which could lead to the explicit incorporation of enzymes in biogeochemical models, improving upon the ability of current models to predict rates of biogeochemical processes in dynamic environments. (C) 2012 Elsevier Ltd. All rights reserved.