Characterising the temperature dependence of anaerobic CH₄ and CO₂ production from intact and drained New Zealand peatlands

Abstract

Understanding the temperature response of anaerobic microbial processes in wetlands is important in determining consequences for carbon dynamics and the production of methane (CH₄) under climate warming scenarios. Natural wetlands, including peatlands, contribute roughly 30% of global CH₄ emissions, making them the largest natural source of CH₄. The transfer of carbon-based greenhouse gases (C-GHG) to the atmosphere produced from the significant amounts of organic matter stored in anaerobic environments could cause a positive feedback to climate warming. This is concerning for peatlands as these environments hold enormous global carbon stocks and given projected global temperature increases. The consensus of previous work suggests that climate warming will result in the acceleration of organic matter decomposition, stimulating CH₄ and carbon dioxide (CO₂) emissions from peatlands. However, the poorly constrained temperature response of CH₄ production continues to plague ecosystem models, due to a lack of understanding of the parameters controlling this process. Despite the importance of wetlands as global sources of CH₄, there is uncertainty among CH₄ production versus temperature models in these environments. Verifying models over a larger temperature range enables us to extract information from a dataset and capture important features such as the temperature optimum or point of rate decline. Therefore, to extract meaningful information on the temperature response of wetland methane production, we collected data on the response across a large temperature range to capture the full curvature of the response. We generated detailed temperature response curves of CH₄ production from two New Zealand peatlands. As far as we are aware, these data are unique in the international literature and provide new information for interpreting ecosystem-scale fluxes. To improve our understanding, we quantified the temperature response of anaerobic organic matter decomposition into CH₄ and anaerobic CO₂ for both an intact and a drained New Zealand peatland. We also compared anaerobic CH₄ and CO₂ production rates across different vegetation types by examining different locations within the intact site. Understanding the difference in C-GHG production potential from contrasting land uses has important implications for managing peat soils and when considering the effects of rewetting drained systems and for comparing the relative net climate forcing of each ecosystem. We developed a methodology for anaerobic peat sampling and performed laboratory incubations for both intact and drained peat. Peat samples were incubated for four days in a temperature gradient block ranging from 8.5–51°C, with 18 discrete temperatures that allowed three replicates at each temperature. Using Macromolecular Rate Theory (MMRT), we derived temperature response metrics for anaerobic CH₄ and CO₂ production, including the temperature optimum (Tₒₚₜ) and the inflection point (Tᵢₙբ). The Tₒₚₜ for CH₄ production ranged from 30.1–32.8°C and Tᵢₙբ values ranged from 23.3–25.6°C. The temperature response did not significantly differ between sites for CH₄, and each peak was relatively tightly constrained, with a sharp increase in production rates at around 15–20°C, followed by a rapid decline in rates above the Tₒₚₜ. On the other hand, the anaerobic CO₂ production curves were less constrained by temperature and differed across sites. Both Tₒₚₜ and Tᵢₙբ were higher for anaerobic CO₂ compared to CH₄, with Tₒₚₜ ranging from 35.4- 44°C and Tᵢₙբ ranging from 25–32.2°C. Overall, this study observed that anaerobic CH₄ and CO₂ production in intact and drained peatlands in New Zealand showed a response with temperature that was well described using MMRT. Despite differences in curvature, temperature metrics did not significantly differ between sites. Anaerobic CO₂ was produced at much higher rates than CH₄, however, the source of the CO₂ is unclear. Additionally, the molar ratio of CO₂:CH₄ increased dramatically above methanogenic Tₒₚₜ. This may provide a positive feedback to the carbon cycle and have important implications for the rewetting of drained peat.