The Michaelis-Menten kinetics of soil extracellular enzymes in response to temperature: a cross-latitudinal study

Decomposition of soil organic matter (SOM) is mediated by microbial extracellular hydrolytic enzymes (EHEs). Thus, given the large amount of carbon (C) stored as SOM, it is imperative to understand how microbial EHEs will respond to global change (and warming in particular) to better predict the links between SOM and the global C cycle. Here, we measured the MichaelisMenten kinetics [maximal rate of velocity (Vmax) and half-saturation constant (Km)] of five hydrolytic enzymes involved in SOM degradation (cellobiohydrolase, beta-glucosidase, beta-xylosidase, a-glucosidase, and N-acetyl-beta-d-glucosaminidase) in five sites spanning a boreal forest to a tropical rainforest. We tested the specific hypothesis that enzymes from higher latitudes would show greater temperature sensitivities than those from lower latitudes. We then used our data to parameterize a mathematical model to test the relative roles of Vmax and Km temperature sensitivities in SOM decomposition. We found that both Vmax and Km were temperature sensitive, with Q10 values ranging from 1.53 to 2.27 for Vmax and 0.90 to 1.57 for Km. The Q10 values for the Km of the cellulose-degrading enzyme beta-glucosidase showed a significant (P = 0.004) negative relationship with mean annual temperature, indicating that enzymes from cooler climates can indeed be more sensitive to temperature. Our model showed that Km temperature sensitivity can offset SOM losses due to Vmax temperature sensitivity, but the offset depends on the size of the SOM pool and the magnitude of Vmax. Overall, our results suggest that there is a local adaptation of microbial EHE kinetics to temperature and that this should be taken into account when making predictions about the responses of C cycling to global change.