Minimal thermal adaptation of soil extracellular enzyme activities along a long-term geothermal gradient

Abstract Changes in soil nutrient cycling with warming are dependent on differing thermal responses and adaptation strategies of soil microbial communities and the interactions between substrate acquisition through extracellular enzyme activity and utilisation via respiration, metabolism and biomass accumulation. Understanding these processes is complicated by the complexity and variability of soil systems, along with differences in methodologies quantifying adaptation of soil microbial processes. Here we aimed to investigate thermal adaptation for extracellular enzyme activity of three enzyme classes (β-glucosidases, β-N-acetylglucosaminidases and phosphatases) using soils from a field setting with different mean annual temperatures across a long term (> 20 year) geothermal gradient in New Zealand. Extracellular enzyme thermal responses showed increasing rates up to the highest characterised temperature (60 °C) regardless of mean annual soil temperature and minimal adaptation across the gradient for parameters such as activation energy, curvature, and the optimum and minimum temperature of activity. These results are in direct contrast to the previously measured thermal response of respiration and growth rates measured at this site which have an optimum temperature of activity around 30–45 °C and show measurable rates of adaptation across the thermal gradient. This divergence in the thermal response of soil substrate depolymerisation via extracellular enzyme activity compared to respiration and growth rates raises questions around future nutrient bioavailability and utilisation if these two process are decoupled at elevated temperatures under future soil warming.