The Effect of Soil Warming On Extracellular Enzyme Kinetics in a Piedmont Ultisol.

Rising temperatures associated with climate change have the potential to alter carbon efflux from soils, thus fundamentally altering the global climate system. The magnitude and direction of this change is largely dependent on the temperature sensitivity of soil organic matter (SOM) decomposition, which proceeds through the activity of microbially-produced extracellular enzymes. The Arrhenius equation predicts that the rate of any enzyme-catalyzed reaction should increase with rising temperatures. However, the temperature sensitivity of extracellular enzymes involved in decomposition may vary due to the production of alternate isozymes with different Vmax. In this study, we assessed how temperature sensitivity changed in response to soil warming in a highly-weathered Georgia ultisol soil, across three seasons. Soil warming (+3 or +5 °C above ambient) was conducted in two habitats: closed-canopy, and canopy gap. We assayed Vmax of four enzymes, β-glucosidase (BG), β-xylosidase (BX), β-D-cellubiosidase (CB), and N-acetylglucosaminidase (NAG), at three different incubation temperatures (15, 25, and 35 °C). From the Arrhenius equation we calculated apparent activation energies (Ea) and Q10s (a measure of relative temperature sensitivity) for each enzyme, soil, and season combination. We found that the apparent Ea and Q10 of BG decreased with soil warming treatment, suggesting enzyme acclimation of the microbial community, while BX, CB, and NAG showed no significant change in apparent enzyme kinetics with soil warming. Our findings suggest that warming can trigger a response in enzyme kinetics, either through shifts in the composition of soil microbial communities or through acclimation of individual organisms via the expression of alternative isozyme pathways. By considering such shifts in enzyme kinetics, we can improve our mechanistic understanding of how soil-carbon will respond to warming climate.