Activation Energy of Extracellular Enzymes in Soils from Different Biomes

<div><p>Enzyme dynamics are being incorporated into soil carbon cycling models and accurate representation of enzyme kinetics is an important step in predicting belowground nutrient dynamics. A scarce number of studies have measured activation energy (E_a) in soils and fewer studies have measured E_a in arctic and tropical soils, or in subsurface soils. We determined the E_a for four typical lignocellulose degrading enzymes in the A and B horizons of seven soils covering six different soil orders. We also elucidated which soil properties predicted any measurable differences in E_a. β-glucosidase, cellobiohydrolase, phenol oxidase and peroxidase activities were measured at five temperatures, 4, 21, 30, 40, and 60°C. E_a was calculated using the Arrhenius equation. β-glucosidase and cellobiohydrolase E_a values for both A and B horizons in this study were similar to previously reported values, however we could not make a direct comparison for B horizon soils because of the lack of data. There was no consistent relationship between hydrolase enzyme E_a and the environmental variables we measured. Phenol oxidase was the only enzyme that had a consistent positive relationship between E_a and pH in both horizons. The E_a in the arctic and subarctic zones for peroxidase was lower than the hydrolases and phenol oxidase values, indicating peroxidase may be a rate limited enzyme in environments under warming conditions. By including these six soil types we have increased the number of soil oxidative enzyme E_a values reported in the literature by 50%. This study is a step towards better quantifying enzyme kinetics in different climate zones.</p> </div

Soil characteristics and environmental variables.

<p> <i>Total C = total carbon, avg T = average air temperature (°C) for the month preceding sampling, GWC = gravimetric water content, MAT = mean annual temperature, MAP = mean annual precipitation.</i></p

Linear and polynomial regression statistics relating E_a for four enzymes to four different environmental variables.

<p> <i>Data are shown for A and B horizon regressions in Supplemental </i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059943#pone-0059943-g001" target="_blank"><i>Figure 1</i></a><i>, avg T = average air temperature (°C) for the month preceding sampling; MAT = mean annual temperature (°C).</i></p

Hydrolytic and oxidative enzyme activities in the A horizon.

<p>Enzyme activities at (a) 4°C, (b) 21°C, and (c) 30°C for hydrolytic enzyme activities in order by average air temperature 30 days prior to sampling. Oxidative enzyme activity at (d) 4°C, (e) 21°C, and (f) 30°C in order by average air temperature 30 days prior to sampling.</p

Activation energy for hydrolytic and oxidative enzymes.

<p>E_a in (a) A horizon and (b) B horizon for β-glucosidase (BG), cellobiohydrolase (CB), peroxidase (PER) and phenol oxidase (POX). Activation energy calculated from three subsamples taken from each soil and depth combination. The number of study locations per biome were tropical n = 2; temperature n = 3; subarctic n = 1; arctic n = 1. The “*” indicates a significant effect of biome with a 0.05≤<i>P</i>≤0.1 and “**” indicates a significant effect of biome with a <i>P</i>≤0.01.</p

Activation Energies (kJ mol⁻¹) with standard error in parentheses (analytical replicates n = 8 for BG, CB; n = 16 for PER, POX) for extracellular enzymes. “*” indicated n = 1.

<p>Activation Energies (kJ mol⁻¹) with standard error in parentheses (analytical replicates n = 8 for BG, CB; n = 16 for PER, POX) for extracellular enzymes. “*” indicated n = 1.</p

Appendix A. Mean cumulative respiration (µg CO2-C/g soil) over successive 60-d periods for control sites incubated at constant temperature and soils that underwent incubation experimental warming after 60, 150, 300, and 450 days.

Mean cumulative respiration (µg CO2-C/g soil) over successive 60-d periods for control sites incubated at constant temperature and soils that underwent incubation experimental warming after 60, 150, 300, and 450 days