Year

2023

Degree Name

Bachelor of Environmental Science (Honours)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Jeffrey Kelleway

Abstract

Coastal wetlands are long-term carbon sinks capable of sequestering carbon for millennia. However, for coastal wetlands to have a net cooling effect on the atmosphere, their carbon storage must exceed their greenhouse gas (GHG) emissions. Meanwhile, the conditions that facilitate carbon sequestration favour the production of several potent GHGs, including methane (CH4) and nitrous oxide (N₂O). The direction and magnitude of these fluxes may be modulated by multiple environmental drivers, including temperature, salinity, and inundation dynamics. While there is increasing understanding of coastal wetland CO2 fluxes, little research has been conducted on the drivers of CH4 and N₂O, particularly within the Australian context. In summer 2022 and winter 2023, soil-atmosphere fluxes were analysed via Fourier-transform infrared (FTIR) spectroscopy using 12 chambers installed over an elevation gradient spanning mangrove, lower saltmarsh, upper saltmarsh and swamp oak forest ecosystems at Towra Point Nature Reserve, Botany Bay, New South Wales, Australia. These data were compared with water-level loggers installed adjacent to each vegetation community, recording temperature, salinity, and water level to understand the physicochemical drivers of GHG sediment flux variation. Site-wide mean sediment CO2 fluxes (2.96 × 10-05 μg m-2 s-1) were several orders of magnitude larger than both CH4 (-1.56 × 10-08 μg m-2 s-1) and N₂O (-2.12 × 10-09 μg m-2 s-1), even when adjusted for global warming potential (GWP). Vegetation communities acted as CH4 and N₂O sources and sinks in different seasons. A two-way ANOVA was used to examine the effects of vegetation community and season on fluxes, and a multiple linear model to explore the physicochemical drivers of flux variation. Generally, GHG fluxes were higher in summer than winter, likely due to enhanced microbial activity in warmer sediment. As expected, the low elevation mangrove community was a moderate CH4 source due to the anoxic conditions that facilitate anaerobic respiration. In contrast, the lower and upper saltmarsh acted as mean CH4 sinks (-6.31 × 10-08 and -5.008 × 10-08 μg m-2 s-1 respectively). Sediment N₂O fluxes were complex and, in some cases, switched from sources and sinks between seasons. Significant upper saltmarsh summer sediment N₂O efflux was observed (2.24 × 10-08 μg m-2 s-1). Variation in the sediment flux of oxygen-containing molecules (i.e., CO2 and N₂O) was partly explained by changes in temperature and salinity. In contrast, variation in CH4 sediment flux was driven primarily by inundation dynamics. The opposite behaviour of oxygen and non-oxygen molecules in response to physicochemical variation highlights the unique electron structure and high electronegativity of oxygen and the important role it plays in redox reactions which are central to wetland biogeochemical processes. These findings provide insight into the magnitude of intertidal wetland soil GHG fluxes and the drivers of flux variation. With sea levels predicted to rise by 0.43-0.84m by 2100, and coastal wetlands already undergoing significant structural change, it is critical that we understand how coastal GHG emissions will respond to a warming climate.

FoR codes (2020)

410604 Soil chemistry and soil carbon sequestration (excl. carbon sequestration science)

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.