Wetlands and the carbon cycle

Carbon is the main building block of all life on earth. The carbon cycle refers to the myriad processes by which natural systems absorb and emit carbon. Plants and animals use carbon to build their cell structures. Carbon can also be deposited in soils. This is how carbon is absorbed or ‘sequestered’. Stored carbon can be released or emitted through natural respiration or when cell structures decompose, are burnt, or in the case of soil carbon, are disturbed.

Prior to the Industrial Revolution, the carbon cycle processes of emitting and sequestering were generally balanced with vast amounts of carbon being trapped over many millennia in highly condensed forms such as coal, oil and natural gas, also known as fossil fuels.

Since the Industrial Revolution, the carbon content of fossil fuels is being released or emitted as those fuels are burnt for generating energy and manufacturing. Also, the natural sequestering processes, especially those performed by plants, are being disrupted as land is cleared for human needs. These carbon emissions are a major driver of the greenhouse effect and human-induced climate change.

Wetlands play a dynamic and crucial role in absorbing atmospheric carbon[6][2][5]. According to the Ramsar Scientific and Technical Review Panel, wetlands cover just 9% of the planet’s land surface, yet are estimated to store 35% of terrestrial carbon.

Figure 1: Queensland Wetlands Program 2012

Generally, undisturbed, or intact wetlands tend to act as ‘carbon sinks’ due to their dense vegetation, algal activity and soils. They regulate processes such as anaerobic decomposition which generates methane and nitrous oxide. These gases respectively have 21 and 310 times more global warming impact than carbon dioxide over a 100 year timeframe[4][7]. However, the capacity of intact wetlands to absorb and sequester carbon varies widely depending on the type of wetland, temperature and water availability[5][2][3]. Research continues to better understand and identify the processes involved, and how these processes will be affected by climate changes.

As wetlands generally act as carbon sinks, their drainage and destruction can result in substantial carbon emissions[7]. Taking water out of a wetland means oxygen can reach previously inundated organic matter. This results in large emissions of carbon dioxide as the organic matter oxidises. Thus, the degradation of Queensland’s wetlands, especially melaleuca wetlands and mangroves, contributes to overall greenhouse gas emissions[3].

Of particular concern is the drainage and disturbance of potential acid sulphate soils, many of which underlay wetlands. When exposed to air these soils release large amounts of greenhouse gas emissions[4].

Australia’s wetlands are atypical due to the country’s dryness and climate variability. In many countries, peatlands play an important role in carbon storage, but these habitats are relatively rare in Australia. Coastal wetlands, especially tidal marshes, mangroves and seagrass are considered to have the greatest potential as carbon sinks in Australia. This is sometimes called blue carbon. It has been estimated that every hectare of mangrove wetland stores about 550t of carbon.

Restoring and protecting wetlands presents an important opportunity for mitigating greenhouse gas emissions[7]. Globally, there is significant work underway to develop methodologies for restoring and managing wetlands that will support the generation of carbon credits. In Australia, work is underway to develop a Blue Carbon accounting methodology under the Commonwealth Government’s Carbon Farming Initiative.

Research supported by Queensland Government as part of the Land Restoration Fund and conducted by Deakin University’s Blue Carbon Lab has modelled blue carbon soil stocks for seagrass and mangroves in Great Barrier Reef Catchments.

Additional links

• Carbon sequestration by Victorian inland wetlands
• The Extraordinary Story of Peat and Carbon
• Wetlands and Changes in Climate
• Regrowth benefits tool
• Towards an Emissions Reduction Fund Method for Blue Carbon
• Technical review of opportunities for including blue carbon in the Australian Government’s Emissions Reduction Fund
• Australian Government initiatives for blue carbon

References

1. ^ Blue Carbon in the Great Barrier Reef catchment. [online] Available at: https://deakin.maps.arcgis.com/apps/Cascade/index.html?appid=b0cc86bd5dfb4dac8cc2d1ce2d8303b7 [Accessed 1 September 2020].
2. ^ a b Coletti, JZ, Hinz, C, Vogwill, R & Hipsey, MR (2013), Hydrological controls on carbon metabolism in wetlands’, Ecological Modelling, vol. 249, pp. 3-3-18.
3. ^ a b Commonwealth of Australia (2012), Issues Paper: The Role of Wetlands in the Carbon Cycle. [online] Available at: http://www.environment.gov.au/resource/issues-paper-role-wetlands-carbon-cycle.
4. ^ a b Hicks, WS, Bowman, GM & Fitzpatrick, RW (1999), Environmental Impact of Acid Sulphate Soils near Cairns, Qld, vol. 15 (99), CSIRO Land and Water Technical Report.
5. ^ a b Junk, WJ, An, S, Finlayson, CM, Gopa, B, Kvet, J, Mitchell, SA, Mitsch, W & Roberts, RD (2013), 'Current state of knowledge regarding the world’s wetlands and their future under global climate change: a synthesis', Aquatic Science. [online], vol. 75, pp. 151-151-167. Available at: https://link.springer.com/article/10.1007/s00027-012-0278-z.
6. ^ Liu, Y, Ni, H, Zeng, Z & Chai, C (2013), Effect of disturbance on carbon cycling in wetland ecosystem’, Advanced Materials Research, pp. 3186-3186-3191.
7. ^ a b c Page, KL & Dalal, RC (2011), '1.   Contribution of natural and drained wetland systems to carbon stocks, CO2, N2O, CH4 fluxes: An Australian perspective', Soil Research. [online], vol. 49, no. 5, pp. 377-377-388. Available at: http://www.publish.csiro.au/?act=view_file&file_id=SR11024.pdf.

Last updated: 10 September 2020