Temperature Sensitivity of Soil Respiration

Abstract

Biogeochemical cycles, such as the carbon (C) cycle, are being continuously affected by anthropogenic activities such as carbon dioxide (CO₂) release from fossil fuels and the decomposition of soil organic C following some land management practices. The C cycle consists of four C pools: atmospheric CO₂, biota (mostly in vegetation), the ocean and soil organic matter (SOM), this being the largest actively cycling C pool. Respiration is the main driver of CO₂ release into the atmosphere, and is extremely sensitive to changes in moisture and temperature with small changes in these variables having a major influence on C cycling. Studies have argued in favour of both positive and negative feedbacks between CO₂ and global warming, where increasing temperatures could either increase CO₂ production, or enhance C storage. These contradicting arguments about how temperature changes will affect C exchanges, makes the understanding of how temperature change will affect temperature sensitivity of the C cycle dynamics (including respiration) critical for C modelling and budgeting. Studies have attempted to measure temperature sensitivity of respiration when trying to understand soil response variation in temperatures. However, synthesis of current literature highlighted that laboratory methods can be problematic and introduce artefacts that obscure true temperature sensitivity. Problems highlighted included, too few temperatures treatments at which respiration was measured, use of long-term incubations that do not control microbial adaptation, lack of seasonal measurements and accounting for variability in moisture content. A new laboratory method was developed for this thesis, which allowed for rapid determination of soil respiration rate at a wide range and number of temperatures to overcome some of the observed drawbacks frequently seen in the literature. A temperature block allowed simultaneous measurements of soil respiration rates over five hours at 44 different temperatures between ~4 and 50 °C. The objective of this thesis was to test this method on a variety of conditions (including different soil types, sampling season, range of moisture contents and pre-incubation temperatures) to understand how temperature sensitivity (Tm_sens) of soil respiration might change with different sample collection and processing approaches. Seasonal measurements of respiration rates from different soil types collected from a single farm found a significant interaction for Tm_sens between soil type and season over the year. This interaction indicated that temperature sensitivity of soil respiration differed between soil types depending on season and thus in order to assess temperature sensitivity of soil respiration from a site, samples should be taken from all soil types at the location. Tm_sens was not dependent on season alone suggesting that a single sampling per year may be sufficient to estimate temperature sensitivity for a soil type, at least at a site with moderate changes in temperature during the year. Surprisingly, respiration rate response to temperature was not very sensitive to variations in soil moisture. Pre-incubated soils sampled for 10 months at different temperatures similarly resulted in no significant change in Tm_sens suggesting that temperature sensitivity of soil respiration can be accurately determined using soils stored at various temperatures and that microbial populations are relatively stable in response to incubation temperature. Overall the developed method was able to rapidly assess temperature sensitivity of several soils through time under a variety of treatments and suggested overall the microbial population did not change rapidly and retained its temperature sensitivity. The success of this method allows for future testing of other hypothesis with regard to temperature sensitivity of respiration.