Chemical and Microbial Controls on the Temperature Sensitivity of Heterotrophic Soil Respiration.

Present concepts emphasize that temperature sensitivity of heterotrophic soil respiration (Rh) depends on substrate quality. Temperature not only controls quality-dependent soil organic matter destabilization but equally the physiology of soil microorganisms and these two are difficult to distinguish. To separate destabilization from physiology effects experimentally, we measured activation energies (Ea) during controlled destabilization by combustion and microbial respiration of mineral and organic soils. Under ambient conditions, enzymes greatly reduce Ea from 136 to 87 kJ mol^-1; thereby increasing CO₂ release by a factor of 1.5 E+7. Temperature dependencies of chemical and microbial oxidation are mutually independent, suggesting weak control of chemical recalcitrance on Ea_Rh. Microbial Ea is highest in soils with wide C/N ratio, low pH, and is caused by declining substrate-use efficiencies with temperature. In the event of warming, ecosystems with poor substrate quality will probably emit more CO₂ and convert less litter into stabilised soil organic matter.