School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemMethane oxidation in soil from the Bogong High Plains, Victoria: controlling factors and fire effects( 2014)Fire rapidly changes terrestrial ecosystems by removing vegetation and exposing mineral soil. Methane (CH4) is an important greenhouse gas but little is known about the effects of fire on its production or consumption. Aerobic soils, such as those found in temperate forests and woodlands are important sinks of CH4, consuming 15-45 Tg y-1 of global atmospheric CH4. The composition and activity of the methanotrophic bacteria responsible for this sink are affected by disturbances such as fire. In this study, sub-alpine sites in the Bogong High Plains, Victoria, were used to examine interacting effects of climate and abiotic soil properties on CH4 consumption. Two predominant vegetation types, Alpine Ash (Eucalyptus delegatensis) forest and Snow Gum (E. pauciflora) woodland, were selected to encompass a range of recent fire histories. Methane oxidation followed first-order enzyme kinetics in Alpine Ash soil so that rates were exponentially proportional to the concentration of available CH4. Methane concentrations decreased exponentially down the soil profile indicating that the sole source of CH4 for oxidation was diffusion from the atmosphere. Soil at depths 5 to 20 cm had the greatest capacity to oxidise CH4. The Michaelis-Menten model was used to describe the kinetics of the enzyme rate reaction of CH4 oxidation, and the model parameters indicated that the bacteria responsible were high-affinity methanotrophs (Type II). Soils were confirmed as CH4 sinks with oxidation rates averaging 76.9 ± 5.3 µg CH4 m-2 h-1 in Alpine Ash forest, and 84.2 ± 4.9 µg CH4 m-2 h-1 in Snow Gum woodland. The effects of key abiotic properties such as CH4 diffusion, soil moisture, temperature, bulk density, inorganic nitrogen and pH were examined in relation to potential variation in CH4 oxidation associated with climate change and fire. Soil moisture influenced CH4 oxidation such that at high moisture content, CH4 oxidation was limited by diffusion, and at low soil moisture content methanotroph activity was limited by physiological water stress. Temperature had a relatively small effect with marginal linear increases in CH4 oxidation in the temperature range of 5 to 30 C°C. Methane oxidation was exponentially limited by increasing soil ammonium concentrations, which had a greater effect on CH4 oxidation than temperature and pH. This indicated a potential impact of fire, as soil ammonium concentrations increased in the first year after fire. The relationship between soil pH and CH4 oxidation was quadratic, with CH4 oxidation decreasing with changes in pH either side of an optimum. Soil pH increased after fire although sites that were more recently or severely burnt were still below the optimum pH for CH4 oxidation. This study provided an understanding of the variation of CH4 oxidation in soil from sub-alpine vegetation in the Bogong High Plains. Further research is required to estimate the magnitude of the soil CH4 sink in the Bogong High Plains, and to predict net changes in CH4 fluxes under future fire and climate scenarios, including improved understanding of the composition of methanotroph populations responsible for this important CH4 sink.