In situ experimental warming has few effects on the temperature sensitivity of hydrolytic soil enzyme Michaelis-Menten kinetics in two southern boreal forest ecosystems

Background/Question/Methods

The decomposition of soil organic matter is expected to increase with climate warming, which could release large amounts carbon stored in soils to the atmosphere as CO₂. Soil extracellular enzymes are important catalysts of organic matter decomposition, as they break down organic polymers and increase the carbon substrates available to the soil microbial community. Soil microbial communities are expected to thermally adjust to higher temperatures by down-regulating enzyme production, and producing isoenzymes, which together could reduce the warming enhancement of the soil carbon decomposition.

We examined the effects of warming on four hydrolytic extracellular enzymes important for soil carbon and nitrogen decomposition. Soils were sampled three times across the snow-free season in the fourth year of in situ warming (+3.4°C) at the B4WarmED project, a highly replicated open-air warming experiment located at two southern boreal forests (~150 km apart) in Minnesota, USA. We measured the Michaelis-Menten kinetics (and their temperature response) for these soil extracellular enzymes. We expected that soil microbial community responses to warming would decrease the maximum enzymatic rate (V_max) and Michaelis-Menten constant (K_m), the binding affinity between substrate and enzyme, as well as the temperature sensitivity of these kinetic parameters, when measured at a common laboratory temperature.

Results/Conclusions

Contrary to predictions, in situ experimental warming increased V_max and the temperature sensitivity of V_max of N-acteyl glucosaminidase. For the kinetics of α-glucosidase, β-glucosidase, and cellobiohydrolase, warming effects differed by site and season, and included both positive and negative effects of warming on kinetic parameters and their temperature sensitivity. These results indicate that the activity and response to temperature for four soil extracellular enzymes important in decomposition are not reduced by warming, suggesting that changes in soil enzymes are unlikely to moderate warming response of soil carbon decomposition.