Carbon Dynamics and Water Quality in Australian Mountain Peatlands: A Comparison of Ecohydrologically Intact and Degraded Systems
Creators: name: Jayasekara Mudiyanselage, Charuni Kanchana Jayasekara
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 410101
subject: Carbon sequestration science

description: Peatlands are a type of terrestrial wetland ecosystem that forms in areas where water inputs exceed water outputs. Peatlands only occupy ~3% of Earth’s terrestrial land area but hold ~30% of the world’s soil carbon. Intact peatlands have a near-surface water table that prevents the aerobic decomposition of organic materials and promotes methanogenesis, thus acting as net CO2 sinks and CH4 sources. However, the degradation of peatlands due to anthropogenic activities and climate change leads to lowered water table levels in peatlands and disrupts the peatland carbon balance, converting peatlands into net CO2 sources. Peatlands are rare in Australia due to the dry climate, but the small number of peatlands that exist in the southeastern Alps of the Australian mainland play a significant role in hydrological regulation in upper catchment areas of the major rivers in the Murray-Darling basin, Australia’s largest river system. The degradation of Australian mountain peatlands started with European colonisation in the 1800s, and cattle grazing continued to damage peatlands until it was completely banned in 2006. The carbon balance and the ecohydrological and meteorological variables that govern the carbon balance in Australian mountain peatlands, whether degraded or otherwise, are still poorly understood. Therefore, the overall aim of my study is to compare the CO2 fluxes, peat decomposition rates, and water quality between two ecohydrologically different Australian mountain peatland ecosystems, one assessed as in intact condition and one in degraded condition.

The first objective of my study was to investigate the effects of substrate quality, peat depth, temperature, peat water content and peatland condition on organic matter decomposition rates. Organic matter decomposition rates in degraded and intact Australian mountain peatlands were measured by conducting field and laboratory incubation experiments. The field incubation experiment was conducted by incubating two types of peat substrate (moss-dominated and partially decomposed) at three different depths (5, 15, and 30 cm) in intact and degraded peatlands. The laboratory incubation experiment was conducted by incubating two peat substrates (moss-dominated and partially decomposed) collected from three depths (0 – 5, 5 – 15, and 15 – 30 cm) that were adjusted to four water contents (50%, 400%, 1500%, and field-moist) at four different temperatures (7, 14, 21, and 28 °C). The results of my incubation experiments indicated that the relatively high water table levels in the degraded peatland reduce peat decomposition rates due to anaerobic conditions within peat layers. Moss-dominated peat decomposes faster than partially decomposed peat under aerobic conditions due to the higher percentage of bioavailable carbon. Furthermore, in moss-dominated peat, water availability acted as the limiting factor for peat decomposition under high temperatures, while in the partially decomposed peat, increasing peat depth limited the decomposition rate under high temperatures.

The second objective was to investigate NEE and identify the main factors affecting trends in NEE. The CO2 flux of the intact peatland was continuously measured using an Eddy Covariance flux tower installed at the peatland, while the CO2 and CH4 flux of the degraded peatland was measured seasonally using the chamber method. The intact peatland was an annual net CO2 sink. The CO2 uptake in the growing season was strongly correlated with photosynthetic photon flux density, while in the non-growing season, soil temperature was the main factor affecting CO2 emissions. The moss-present areas of the degraded peatland were net CO2 sinks, and the moss-absent areas were net CO2 sources, likely due to vascular plant-dominated vegetation and degraded peat in the moss-absent areas. The regular seasonal water table variations did not affect the CO2 flux in either peatland, but occasional very low water table levels induced by severe drought significantly increased CO2 emissions from both peatlands, reducing net CO2 sequestration. The degraded peatland was a net CH4 source during the growing season, but CH4 emission values were almost half those of the intact peatland due to low substrate quality in degraded peat.
The third objective of quantifying surface and pore water chemistry and identifying any differences related to peatland condition was investigated by analysing seasonal surface and pore water samples for dissolved organic carbon and nitrogen, anions, cations, iron concentrations and characteristics of dissolved organic matter. The intact peatland had a strong seasonality and high nutrient removal due to its higher productivity than the degraded peatland. The oxidation potential and dissolved organic carbon values did not differ between the two peatlands due to the high water table levels at both sites. However, a predominance of degraded catotelm peat in the degraded peatland resulted in higher amounts of dissolved organic nitrogen and heavily degraded and humified dissolved organic matter in the water than in the intact peatland.

The final objective of this study was to determine the suitability of NEE as an indicator of peatland condition and ecosystem health, drawing upon peatland NEE studies from around the globe. A systematic quantitative literature review was conducted, using NEE values for 130 peatlands, collated from 60 primary research articles. The results of this study indicated that the majority of intact peatlands are net CO2 sinks, degraded peatlands tend to be net CO2 sources, and most restored peatlands convert to net CO2 sources from sinks. Therefore, I concluded that net ecosystem exchange could be applied as one of the useful indicators of peatland condition and restoration progress.
 
The results of this study have important implications for the management and restoration of degraded peatlands. Overall, the findings demonstrate that increasing water table levels by blocking drainage lines would lower oxidation potential, peat decomposition rates, CO2 emissions, and aquatic carbon export from degraded peatlands. Furthermore, the results of this study also suggest that Sphagnum recolonisation will reduce CO2 emissions from degraded peatlands and increase nutrient removal from water moving through these systems. Future research could focus on investigating carbon dynamics in restored and restoring Australian mountain peatlands. This would enable the identification of when and how ecohydrological changes caused by restoration affect carbon dynamics and how to most effectively implement restoration activities to convert degraded peatlands from net carbon sources to sinks.