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Limited changes in microbial respiration along a geothermal gradient

Soil carbon is the largest terrestrial stock of carbon (C) globally. This C stock has the potential to be negatively impacted by global warming through the acceleration of microbial respiration via positive feedback loops. Microbial respiration is a temperature dependent process that releases carbon dioxide (CO₂). Thermal adaptation of microbial respiration may partially offset positive feedback loops, alleviating accelerated responses. The first aim of this thesis was to determine the temperature response of microbial respiration (of labile C and soil organic matter (SOM)) in soil and evaluate the potential for thermal adaptation using a geothermal gradient as a proxy for soil warming. Here, the geothermal gradient, located in Rotorua, New Zealand, spanned average soil temperatures of 18-36 °C, encompassing a range of temperatures experienced in temperate and tropical ecosystems. Soil from along this gradient was sampled and incubated in a laboratory temperature block at 40 different temperatures (~1.8-53 °C) for five hours. For the experiments, 40 control (soil + distilled water) and 40 treatment (soil + glucose solution) tubes were used to separate the SOM and labile C respiration temperature responses. CO₂ concentrations were measured on an Infrared Gas Analyser (IRGA) after five hours. The second aim of the thesis was to determine the temperature response of priming along the geothermal gradient. Soil priming occurs when added labile C substrates in soil promote the acceleration or deceleration of SOM decomposition. This aim was also completed using the temperature block and IRGA for CO₂ measurements, but instead using a δ ¹³C labelled glucose solution to separate the temperature response of priming. These samples were also run on an Off Axis Integrated Cavity Output Spectroscopy (OA-ICOS) instrument to measure the isotopic fraction. A mixing model allowed the separation of SOM, glucose and priming temperature responses. This thesis used a temperature model called Macromolecular rate theory (MMRT) to characterise the temperature responses in terms of temperature optimum (Tₒₚₜ) and temperature inflection point (Tᵢₙ𝒻). Changes in these parameters gave insight into potential responses to warming temperatures. The results of this thesis found evidence for modest thermal adaptation occurring for the Tₒₚₜ of labile C respiration and the Tᵢₙ𝒻 of SOM respiration in response to soil warming. However, these changes were small with changes no larger than 0.198 °C per °C change in environmental temperature (°C °C⁻¹) and 0.263 °C °C⁻¹, respectively. There was no evidence that the Tᵢₙ𝒻 of labile C respiration and the Tₒₚₜ of SOM respiration changed with increasing environmental temperature. The priming results suggested that the temperature response of priming remains constant at different environmental temperatures but differs largely with soil properties. The degree of priming decreased with increasing incubation temperatures (particularly above 40 °C). Overall, the results suggested that thermal adaptation may occur in response to global warming, however, this adaptation is likely minor. This means that feedback loops may be alleviated in response to climate change. However, the modesty of the observed changes may indicate that thermal adaptation may not occur or may not be enough to offset feedback loops. Additionally, the negative relationship of priming with increasing temperature may dampen feedback loops with climate change. Overall, further work is required to fully understand the implications of soil warming on microbial responses.
Type of thesis
van de Laar, A. (2021). Limited changes in microbial respiration along a geothermal gradient (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14591
The University of Waikato
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