Carbon-use efficiency explains thermal acclimation of soil respiration

Background/Question/Methods

Enhanced soil respiration in response to warming may substantially increase atmospheric CO₂ concentrations above expected anthropogenic contributions.  This response, however, depends on the magnitude and mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short-term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability.  We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in soils exposed to warming, or that physiological acclimation reduces temperature-specific respiration rates, and we explored the role of carbon-use efficiency (CUE) as an underlying mechanism.  To test these hypotheses, laboratory incubations were conducted using soil collected during winter and summer from a high-elevation sagebrush steppe site in southeast Wyoming.  Soils were subjected to different levels of temperature and substrate availability in a two-phase experiment consisting of an acclimation phase and a response phase.  Data were analyzed using a model combining a Michaelis-Menten function for labile substrate (LSC) and microbial biomass (MBC) kinetics with a Lloyd and Taylor temperature response function.  Data-model integration was done in a hierarchical Bayesian framework that allowed for the effects of MBC and LSC to be separated from underlying physiological parameters.

Results/Conclusions

Predicted soil respiration rates under common MBC and LSC levels were significantly lower at a given temperature for summer soils than for winter soils, supporting the hypothesis of seasonal thermal acclimation.  The predicted respiration rates at 1.5°C for winter soils were approximately equal to the respiration rates at 16°C for summer soils, suggesting full seasonal acclimation. The underlying mechanisms were higher CUE and lower basal respiration in summer than winter soils. Short-term apparent acclimation was induced by substrate depletion, wherein more rapid substrate depletion at higher temperatures led to a decline in soil respiration after one-month incubation that did not occur at lower temperatures.  In general, MBC was significantly lower at higher temperatures.  This was likely due to decreased CUE with increasing temperatures.  Overall, the mechanisms underlying seasonal thermal acclimation of soil respiration were quite different from the mechanisms underlying short-term thermal acclimation, and these differences have important implications for predicting changes in soil carbon storage. Our understanding of carbon cycle-climate interactions will be improved by incorporating the seasonality and temperature sensitivity of CUE into models of soil carbon cycling.