Assessing thermal acclimation of soil microbial respiration using macromolecular rate theory

Soil heterotrophic respiration is strongly controlled by temperature. Thus, understanding how soil microbial respiration will acclimate to global warming is important for accurate predictions of soil carbon loss. Thermal acclimation of soil respiration has typically been measured using the Q10 temperature coefficient or comparing absolute rates of respiration with varying conclusions. Discrepancies in these findings may be a result of these approaches not accounting for the temperature optima associated with microbial respiration. To address this issue, we periodically measured the temperature response of respiration for soils incubated at 4, 10, 20, and 35 °C for up to 310 days. We measured respiration rates from these soils placed in a temperature block for 5 h at ~ 1 °C increments with temperatures ranging from ~ 4 to 50 °C. To assess thermal acclimation, we used macromolecular rate theory to calculate the temperature optimum (Topt\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{opt}$$\end{document}), the inflection point of the curve (Tinf)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{inf})$$\end{document}, and the change in heat capacity of the transition state (ΔCP‡\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Delta C}_{P}^{\ddagger }$$\end{document}), as a measure of the temperature response. We compared changes in Topt\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{opt}$$\end{document}, Tinf\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{inf}$$\end{document}, and ΔCP‡\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Delta C}_{P}^{\ddagger }$$\end{document} over time between each of the long-term incubation temperatures. We found that Topt\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{opt}$$\end{document} and Tinf\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${T}_{inf}$$\end{document} increased and ΔCP‡\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Delta C}_{P}^{\ddagger }$$\end{document} decreased at higher long-term incubation temperatures after approximately six months. However, these results appear largely driven by changes in carbon availability, suggesting that the temperature response of soil microbial respiration changes only as soil carbon depletes. This novel approach offers a new perspective on how soil microbial communities may acclimate to climate change and may be relevant for modelling of soil carbon losses.