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 β-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 β-glucosidase was determined by assaying for the enzyme in soil slurries at three different temperatures (15, 25 and 35 °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 Q10 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, β-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 β-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 β-glucosidase activity responded primarily to changes in temperature, except when soils were dry, with water potentials