Abstract
With temperatures rising in Arctic regions at double the global average,
decomposition rates and subsequent greenhouse gas release into the
atmosphere are expected to rise, potentially shifting these regions from
carbon sinks to carbon sources. General circulation models predict the
relationship between respiration and temperature using exponential
functions, showing a temperature-independent rate of increase to
decomposition rates with temperature. However, enzymes catalyzing carbon
depolymerization reactions have demonstrated departures from this
predicted increase between 10 and 15 °C. Organic tundra soils were
incubated across this range (4 to 20 °C, 4 °C increments) and harvested
consecutively after carbon losses equivalent to those after 2, 4, 8, and
12 days at 20 °C. During each harvest, β-glucosidase (BG) and
β-xylosidase (BX) activities were measured at both the incubation
temperature and at 20 °C to determine temperature limitations to enzyme
activity and production. While enzyme activities in soils incubated at
different temperatures diverged significantly when measured at different
temperatures, these differences were overcome when measured at 20 °C,
suggesting enzyme activities are more limited than enzyme production.
Two exceptions to this trend were the first harvest of the 4 and 8 °C
incubations, which had significantly higher BG and BX activities when
assayed at 20 °C than soils incubated at higher temperatures, indicating
larger enzyme pool sizes in soils incubated at lower temperatures. For
BG activities, increased enzyme pool size offset low temperature
limitations to enzyme activity, resulting in no difference in measured
rates when the 4 and 8 °C soils were compared with the 12 and 16 °C
soils, respectively. However, in subsequent harvests, BG and BX
activities measured at 20 °C declined and activities at 4 and 8 °C were
significantly lower than in assays at 12, 16, and 20 °C. Declining
activities of these enzymes in the 4 and 8 °C incubations may indicate
limited substrate accessibility or nutrient availability at lower
temperatures due to changes in the relative rates of other decomposition
reactions with temperature. Improving our understanding of the
multifaceted impacts of temperature on decomposition reactions will be
important to better model and predict future carbon release.