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

Abstract Decomposition of soil organic matter ( SOM ) is mediated by microbial extracellular hydrolytic enzymes ( EHE s). Thus, given the large amount of carbon ( C ) stored as SOM , it is imperative to understand how microbial EHE s 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 M ichaelis– M enten kinetics [maximal rate of velocity ( V max ) and half‐saturation constant ( K m )] of five hydrolytic enzymes involved in SOM degradation (cellobiohydrolase, β‐glucosidase, β‐xylosidase, α‐glucosidase, and N ‐acetyl‐β‐ 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 V max and K m temperature sensitivities in SOM decomposition. We found that both V max and K m were temperature sensitive, with Q 10 values ranging from 1.53 to 2.27 for V max and 0.90 to 1.57 for K m . The Q 10 values for the K m of the cellulose‐degrading enzyme β‐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 K m temperature sensitivity can offset SOM losses due to V max temperature sensitivity, but the offset depends on the size of the SOM pool and the magnitude of V max . 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.