Seasonal plasticity in the temperature sensitivity of microbial activity in three temperate forest soils

The temperature sensitivity of soil organic matter (SOM) decomposition has been a source of much debate, given the potential feedbacks with climate warming. Here, we evaluated possible seasonal variation in the temperature sensitivity of microbially mediated soil fluxes related to decomposition (net N mineralization, net nitrification, proteolysis, the maximum velocity ( V max ) of proteolysis, microbial respiration, and the V max of four soil exo‐enzymes) across forests dominated by eastern hemlock ( Tsuga canadensis ), white ash ( Fraxinus americana ), and red oak ( Quercus rubra ) in central Massachusetts, USA. We asked two simple questions: (1) do temperature sensitivities vary across forest types or different steps of the decomposition process, and (2) do temperature sensitivities display plasticity on a seasonal time frame? We observed substantial variation in temperature sensitivities ( Q 10 and R 10 values) across the different fluxes and forest types. The ash soils exhibited the strongest temperature sensitivities and the mineral‐N fluxes exhibited higher temperature sensitivities relative to the proteolytic fluxes or microbial respiration. The V max of soil exo‐enzymes varied considerably in an interactive manner across forests and time, and the response of some enzymes was consistent with the thermal plasticity. The enzymatic kinetic properties V max and K m (half‐saturation constant) were strongly correlated with slopes that differed across enzymes, reflecting an enzyme‐specific tradeoff between maximum catalytic rate and substrate‐binding efficiency. Generally, Q 10 values were largely constant, but R 10 values varied in a manner consistent with distinct seasonal plasticity. There was a consistent seasonal shift in R 10 values coincident with snowmelt, suggesting that the time following snowmelt is a particularly interesting and dynamic period of microbial activity in these temperate forests.