Understanding ecohydrological and biogeochemical processes in an Australian alpine sphagnum peatland and implications for management under a changing climate

Australia’s alpine Sphagnum peatlands have significant importance as nationally endangered ecosystems. However, key elements of the peatland carbon and hydrological process are not thoroughly understood, which limits the effectiveness of management strategies. Evaluating the reference conditions of a healthy peatland ecosystem would be a useful benchmark for effective rehabilitation and sustainable ecosystem management, as well as enabling the implementation of a long-term reliable monitoring programme. This research’s primary objective was, therefore, to establish the baseline ecohydrological and biogeochemical condition of a healthy mountain peatland ecosystem to serve as a reference ecosystem. The specific objectives of the project are to answer the following questions: 1. What is the potential of the peatland to act as a net carbon sink, and what are the temporal fluxes of carbon in the peatlands? 2. What is the extent of water loss from the peatland as evapotranspiration, and what are the drivers associated with peatland water losses? 3. What are the major peatland biogeochemical processes under base–flow conditions? 4. What is the water budget for this particular peatland catchment, and how does this vary with time? 5. What is the groundwater contribution to peatland recharge, and how does it contribute to the dry season survivability of the peatland? A multi–disciplinary approach was utilised to achieve these objectives, as outlined below. 1. An eddy covariance system was used to investigate the baseline conditions of critical ecosystem functions. The system was then used to measure the net ecosystem exchange (NEE) of CO2 evapotranspiration and energy exchange over the study period. 2. CH4 flux was measured via the chamber–based method. 3. Hydrological parameters (water table depth, catchment outflow, soil moisture etc.) of the peatland, aquatic carbon fluxes (DOC and DIC), and inorganic irons Cl-, Na+, Ca2+ Mg2+ were measured and assessed over time. 4. The first estimation of net ecosystem carbon balance (NECB) for an intact Alpine Sphagnum peatland in Australia was made. 5. Eddy covariance was used to quantify evapotranspiration and combined with Penman-Monteith-based evapotranspiration to calculate an ‘ecosystem vegetation coefficient’ (KESV). 6. Baseflow evapotranspiration and analyses of Cl-, Na+, Ca2+ and Mg2+ in peatland stream water were used to assess biogeochemical processes. 7. A water balance assessment of an Australian alpine peatland ecosystem to assess the hydrological connectivity and contribution of groundwater to the sustainability of the system was conducted. Important findings from my PhD are outlined below. 1. Overall, the ecosystem was a strong net C sink, storing ~ 292.5 g C m-2 yr-1. The annual NECB exhibited distinct seasonal features, switching from a carbon sink in the growing season (-444.1 g C m−2 Season-1) to a carbon source in the non-growing season (151.6 g C m−2 Season-1). Annual methane fluxes were relatively small (2.4 g CH4– C m-2 yr-1) and contributed <1% to the yearly NECB. The estimated yearly average DOC flux was 8.7 g C m-2 yr-1. DIC concentrations in stream exit waters were always less than in groundwater, and the estimated annual average evasion flux was 18.5 g C m-2 yr-1. 2. Evapotranspiration was shown to be a significant component of the water budget, comprising 26% of annual precipitation. 3. The KESV (Peatland vegetation coefficient) varied seasonally between 0.5 (spring and autumn) and 1.1 (summer). This study identified three distinct phenological stages in peatland productivity and suggested that the application of a single year-round KESV value is not appropriate in mountain peatlands. 4. Strong retention and export mechanisms observed for Na+ and Cl− ions and Ca2+ and Mg2+ ions showed approximate conservative behaviour at high ET. 5. Water balance assessment revealed that peatland has >70% ground and surface water recharge contribution during the snow–free period. This proves that peatland is a critical water–regulating component of high-mountainous landscapes that highly contributes to the stream water supply. 6. A conceptual model was formulated on field observations and numerical simulations to illustrate the hydrological connectivity of the main components in the system. Their response to a changing climate will impact all downstream environments and water availability for human use. The reference conditions for the system could be applied as a benchmark to compare against to assess future management policies.