Temperature sensitivity of soil enzyme kinetics under N‐fertilization in two temperate forests

Soil microbes produce extracellular enzymes that degrade carbon (C)‐containing polymers in soil organic matter. Because extracellular enzyme activities may be sensitive to both increased nitrogen (N) and temperature change, we measured the effect of long‐term N addition and short‐term temperature variation on enzyme kinetics in soils from hardwood forests at Bear Brook, Maine, and Fernow Forest, West Virginia. We determined the V_max and K_m parameters for five hydrolytic enzymes: α‐glucosidase, β‐glucosidase, β‐xylosidase, cellobiohydrolase, and N‐acetyl‐glucosaminidase. Temperature sensitivities of V_max and K_m were assessed within soil samples subjected to a range of temperatures. We hypothesized that (1) N additions would cause microbial C limitation, leading to higher enzyme V_max values and lower K_m values; and (2) both V_max and K_m would increase at higher temperatures. Finally, we tested whether or not temperature sensitivity of enzyme kinetics is mediated by N addition. Nitrogen addition significantly or marginally significantly increased V_max values for all enzymes, particularly at Fernow. Nitrogen fertilization led to significantly lower K_m values for all enzymes at Bear Brook, but variable K_m responses at Fernow Forest. Both V_max and K_m were temperature sensitive, with Q₁₀ values ranging from 1.64–2.27 for enzyme V_max and 1.04–1.93 for enzyme K_m. No enzyme showed a significant interaction between N and temperature sensitivity for V_max, and only β‐xylosidase showed a significant interaction between N and temperature sensitivity for K_m. Our study is the first to experimentally demonstrate a positive relationship between K_m and temperature for soil enzymes. Higher temperature sensitivities for V_max relative to K_m imply that substrate degradation will increase with temperature. In addition, the V_max and K_m responses to N indicate greater substrate degradation under N addition. Our results suggest that increasing temperatures and N availability in forests of the northeastern US will lead to increased hydrolytic enzyme activity, despite the positive temperature sensitivity of K_m.