Abstract/Summary

Predictions concerning the feedback of soil heterotrophic respiration to a warming climate often do not differentiate between the extracellular and intracellular steps involved in soil organic matter decomposition. This study examined the temperature sensitivities of intracellular metabolic processes and extracellular soil enzyme activities and how they are influenced by previous temperatures. We pre-incubated soils at 5 °C, 15 °C or 26 °C to acclimatise the microbial communities to different thermal regimes for 60 days before measuring potential activities of β-glucosidase and chitinase (extracellular enzymes), glucose-induced respiration (intracellular metabolic processes), and basal respiration at a range of assay temperatures (5 °C, 15 °C, 26 °C, 37 °C, and 45 °C). A higher pre-incubation temperature decreased soil pH and C/N ratio and decreased β-glucosidase potential activity and respiration, but not chitinase potential activity. It is likely that this legacy effect on β-glucosidase and respiration is an indirect effect of substrate depletion rather than physiological acclimatation or genetic adaptation. Pre-incubation temperature effects on temperature sensitivity were subtle and restricted to extracellular activities, perhaps because of the short (60 day) duration of the pre-incubation at temperatures that were below the initial optimum (~30 °C) for the mesophilic soil community. However, we found that the intracellular and extracellular steps differ in their temperature sensitivity and this observation differs depending on the range of temperature used for Q10 estimates of temperature sensitivity. Between 5 °C and 15 °C intracellular and extracellular enzyme processes show equal temperature sensitivity, but between 15 °C and 26 °C intracellular metabolic processes were more temperature sensitive than extracellular enzyme activity and between 26 °C and 37 °C extracellular enzyme activity was more temperature sensitive than intracellular metabolic processes. This result implies that depolymerisation of higher molecular weight carbon is more sensitive to temperature changes at higher temperatures (e.g. higher temperatures on extremely warm days) but the respiration of the generated monomers is more sensitive to temperature changes at moderate temperatures (e.g. mean daily maximum soil temperature). However, studies using multiple soil types and a greater range of pre-incubation temperatures are required to generalize our results. Nevertheless, since climate change predictions currently indicate that there will be a greater frequency and severity of hot summers and heatwaves, it is possible that global warming may reduce the importance of extracellular depolymerisation relative to intracellular metabolic processes as the rate limiting step of soil organic matter mineralization. We conclude that extracellular and intracellular steps are not equally sensitive to changes in soil temperature and that the previous temperature a soil is exposed to may influence the potential activity, but not temperature sensitivity, of extracellular and intracellular processes.