Coastal wetland soil carbon sequestration revealed from sediment core profiles

Dylan Robert Brown

doi:10.25918/thesis.167

Coastal wetland ecosystems contribute significantly to the global carbon, water and nutrient cycles. In particular, coastal wetlands have recently been noted for their substantial role in mitigating climate change with respect to the implementation of various management practices and their ability to sequester large amounts of carbon from the atmosphere and their tendency to act as natural sinks by accumulating carbon in the soils. Given the predicted global rise in sea levels, however, it is important to consider that these carbon stocks may be remobilised in the near future if left unmanaged. To address some of the major gaps in research knowledge and to contribute to the understanding of the importance of natural wetlands in regards to understanding the carbon pools and associated fluxes that may be important as carbon sources or sinks in relation to the possibilities for anthropogenic management, I used 210Pb to quantify carbon sequestration in differing and previously unaccounted wetlands. Chapter 2 investigates organic carbon accumulation in two coastal acid sulfate soil (CASS) wetlands. The average belowground organic carbon accumulation rates in CASS wetlands (190 ± 20 g m-2 yr-1), derived from 18 210Pb-dated sediment cores, were an order of magnitude greater than terrestrial forests (tropical, boreal, and temperate forests) and within the same range as blue carbon ecosystems (saltmarshes, mangroves, and seagrasses). Considering their large area and carbon accumulation rate, the remediation, management, and adaptation to the prospect of rising sea levels of CASS wetlands may be an important component of climate change mitigation strategies in Australia. Chapter 3 investigates the possibility of hypersaline tidal flats (HTFs) being considered important “blue carbon” systems by quantifying carbon and nutrient burial and atmospheric CO2 fluxes in HTFs in Australia and Brazil. It is estimated that organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) are being buried at rates averaging 21 (± 6), 1.7 (± 0.3), and 1.4 (± 0.3) g m-2 y-1, respectively, during the previous century in three contrasting HTFs systems, one in Brazil (eutrophic) and two in Australia (oligotrophic). Although these rates are lower than those from nearby mangrove, saltmarsh and seagrass systems, the importance of HTFs as sinks for OC, TN and TP may be significant given their extensive coverage. Despite the measured short-term variability between net air-saltpan CO2 influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal CO2 fluxes measurements, the OC sedimentary profiles over several decades suggests efficient OC burial at all sites. These findings highlight a previously unquantified carbon as well as nutrient sink and suggest that coastal HTF ecosystems could be included in the emerging blue carbon framework. Finally, Chapter 4 estimates sedimentary blue carbon stocks from 37 sediment cores collected in pristine (n=13), agricultural (n=11), and urban (n=13) estuaries within the same geomorphological region, located on the eastern coast of Australia. The mean estimated C stocks for each carbon system (seagrass, mangrove and saltmarshes) were 563.6 ± 195.5, 856.3 ± 120.9, and 812.5 ± 63.3 Mg C ha-1, respectively, conservatively estimated up to 3 m depths. Analysis of variance revealed no significant difference between C stocks per area (C ha-1) considering each habitat type and between specific estuaries. However, the total estuarine C stocks were found to be greater with increasing levels of conservation. The implication of these results are that there are large C stocks in small regional estuaries which supports the protection of blue C systems in developing coastal areas and highlights the uncertainties of the CO2 emissions from potential blue C habitat degradation. Overall, the recognition of the carbon sequestration value of wetlands provides a strong argument for their protection and restoration; however differences in wetland type, climatic region and time scale have been identified to influence the potential for wetlands to be net sources or sinks for carbon and therefore need to be considered when providing up to date estimates of carbon sequestration.

Coastal wetland soil carbon sequestration revealed from sediment core profiles

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Coastal wetland soil carbon sequestration revealed from sediment core profiles

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