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

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 B ear B rook, M aine, and F ernow F orest, W est V irginia. 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 F ernow. Nitrogen fertilization led to significantly lower K m values for all enzymes at B ear B rook, but variable K m responses at F ernow F orest. Both V max and K m were temperature sensitive, with Q 10 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 .

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