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Keywords:

  • carbon cycle;
  • decomposition;
  • enzyme kinetics;
  • extracellular enzyme;
  • microbe;
  • nitrogen fertilization;
  • soil warming

Abstract

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 Vmax and Km parameters for five hydrolytic enzymes: α-glucosidase, β-glucosidase, β-xylosidase, cellobiohydrolase, and N-acetyl-glucosaminidase. Temperature sensitivities of Vmax and Km 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 Vmax values and lower Km values; and (2) both Vmax and Km 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 Vmax values for all enzymes, particularly at Fernow. Nitrogen fertilization led to significantly lower Km values for all enzymes at Bear Brook, but variable Km responses at Fernow Forest. Both Vmax and Km were temperature sensitive, with Q10 values ranging from 1.64–2.27 for enzyme Vmax and 1.04–1.93 for enzyme Km. No enzyme showed a significant interaction between N and temperature sensitivity for Vmax, and only β-xylosidase showed a significant interaction between N and temperature sensitivity for Km. Our study is the first to experimentally demonstrate a positive relationship between Km and temperature for soil enzymes. Higher temperature sensitivities for Vmax relative to Km imply that substrate degradation will increase with temperature. In addition, the Vmax and Km 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 Km.