The Temperature Sensitivity of Decomposition of Soil Organic Matter: Moving Beyond Q10.
Eric A. Davidson, The Woods Hole Research Center, 149 Woods Hole Rd, Falmouth, MA 02540-1644
A scientific consensus links human activity, increasing atmospheric carbon dioxide concentrations and global warming, but less agreement exists regarding the effects of climate change on the global carbon cycle. Significantly more carbon is stored in the world's soils, including peatlands, wetlands and permafrost, than is present in the atmosphere. If carbon stored belowground is transferred to the atmosphere by a warming-induced acceleration of its decomposition, a positive feedback to climate change would occur. Conversely, if increases of plant-derived carbon inputs to soils exceed increases in decomposition, the feedback would be negative. Despite much research, a consensus has not yet emerged on the temperature sensitivity of soil carbon decomposition. Numerous lines of evidence from transect studies, laboratory incubations, isotopic analyses, and soil warming experiments indicate that decomposition of young, labile carbon substrates is clearly temperature sensitive, but the results are often equivocal and confusing for older and/or more recalcitrant soil carbon substrates. On the other hand, kinetic theory predicts that the temperature sensitivity of decomposition should increase with increasing activation energies of complex and recalcitrant substrates. Several environmental constraints often obscure the intrinsic temperature sensitivity of substrate decomposition, causing lower observed “apparent” temperature sensitivity. Most Q10 measurements of temperature sensitivity do not consider these environmental constraints, and thus are highly variable. Many of these constraints to decomposition are, themselves, sensitive to climate, and are likely key factors determining future losses of soil carbon. Carbon in high latitude soils is particularly vulnerable to climate-induced removal of constraints to decomposition. Merging concepts of kinetic theory, such the Arrhenius function and Michaelis-Menten kinetics, may allow us to interpret temperature sensitivities within the context of enzyme and substrate concentrations at sites of decomposition reactions, which are controlled by several climate-sensitive environmental constraints.