Temperature Sensitivity of Intracellular and Extracellular Soil Enzyme Activities

Predicting the pattern of soil organic matter (SOM) decomposition as a feedback to climate change, via release of CO₂, is extremely complex and has received much attention. However, investigations often do not differentiate between the extracellular and intracellular processes involved and work is needed to identify their relative temperature sensitivities. Samples were collected from a grassland soil at Sonning, UK with average daily maximum and minimum soil temperature of 15 °C and 5 °C. We measured potential activities of β-glucosidase (BG) and chitinase (NAG) (extracellular enzymes) and glucose-induced CO₂ respiration (intracellular enzymes) at a range of assay temperatures (5 °C, 15 °C, 26 °C, 37  °C, and 45 °C). The temperature coefficient Q₁₀ (the increase in enzyme activity that occurs after a 10 °C increase in soil temperature) was calculated to assess the temperature sensitivity of intracellular and extracellular enzymes activities. Between 5 °C and 15 °C intracellular and extracellular enzyme activities had equal temperature sensitivity, but between 15 °C and 26°C intracellular enzyme activity was more temperature sensitive than extracellular enzyme activity and between 26 °C and 37 °C extracellular enzyme activity was more temperature sensitive than intracellular enzyme activity. This result implies that extracellular depolymerisation of higher molecular weight organic compounds is more sensitive to temperature changes at higher temperatures (e.g. changes to daily maximum summer temperature) but the intracellular respiration of the generated monomers is more sensitive to temperature changes at moderate temperatures (e.g. changes to daily mean summer temperature). We therefore conclude that the extracellular and intracellular steps of SOM mineralisation are not equally sensitive to changes in soil temperature. The finding is important because we have observed greater increases in average daily minimum temperatures than average daily mean or maximum temperatures due to increased cloud cover and sulphate aerosol emission. Accounting for this asymmetrical global warming may reduce the importance of extracellular depolymerisation and increase the importance of intracellular catalytic activities as the rate limiting step of SOM decomposition.