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Peatland (changes)

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Summary

A peatland is an ecosystem type where plant material, usually in marshy areas, is inhibited from decaying fully by acidic and anaerobic conditions. It then forms peat, which is a kind of partially decayed vegetation matter.

Peatlands store more carbon per square metre than any other terrestrial ecosystem. Covering only about 3% of Earth’s land area, they hold the equivalent of half of the carbon that is in the atmosphere as CO2. This makes them important as a potential positive feedback to a warming climate.

There are about 4 trillion m3 of peat in the world, which when dried and burnt might yield about 8 billion terajoules of energy.

Peat structure

Peat is about 95% water, and varies in depth up to 6m or so. See the Peatland Ecology Research Group for more details.

Distribution

Peat bogs are widely distributed in cold, temperate climes, mostly in the northern hemisphere. The world’s largest wetlands are the bogs of the Western Siberian Lowlands in Russia, which cover more than 600,000 km2. Sphagnum bogs were once widespread in northern Europe, and there are extensive bogs in Canada and Alaska (called muskeg). According to the 2007 Survey of Energy Resources, the total area of peatlands approaches 3 million km2 and the total volume of peat in situ is in the order of 3,500 to 4,000 billion m2.

Peat bogs and climate change

Carbon sink, methane source

Partially decomposed organic material accumulates as carbon-rich peat due to an imbalance between the rates of primary production and decomposition, usually because of restrictions on soil oxygen availability, temperature and nutrients. The low oxygen and nutrient levels, waterlogged conditions and cool climate support a unique, characteristic vegetation community dominated by Sphagnum moss.

The same processes of anaerobic decomposition that allow carbon to accumulate also produce the strong greenhouse gas methane (CH4). Over the time span of centuries, peatlands exert a net cooling effect on the global radiation balance, because the effect of removing long-lived atmospheric CO2 ultimately surpasses that of releasing short-lived CH4. However, should peatlands begin to degrade on a large scale, the stored carbon could be released, reducing — or even reversing — their climate cooling effect.

Threats

Climate change

Warmer air temperatures, drier summers and more frequent droughts, have been shown to cause peatlands to degrade and begin to lose, through erosion, decomposition, or fire, the carbon that they have been accumulating for hundreds or thousands of years.

Nitrogen pollution

Nitrogen pollution can cause soil acidification and nutrient enrichment. Moderate increases in nitrogen deposition from a low level may increase plant productivity without an equal increase in decomposition rates, leading to enhanced carbon accumulation. However, shifts in species composition from bryophytes (mosses) to vascular plants (higher plants) may increase the production of easily-decomposable plant material, leading to higher rates of decomposition, and reduced carbon accumulation.

Drainage

The major change to water-table behaviour occurs in the surface (acrotelm) layer only, which may become permanently emptied and result in significant ecological changes. Drainage also causes the lower layer (catotelm) of a peat bog to undergo changes including oxidation of peat carbon. Drained bogs are a substantial carbon source, losing it as gaseous emissions and in water.

Plantation forests on peat

In the short term, conifer forests grown on peat may result in a net carbon gain. In the longer term, however, plantation forests are acknowledged to result in net carbon losses because eventually the carbon gains of the forest are outweighed by losses from the bog. The tipping point, beyond which such conifer forests appear to cause net carbon losses exceeding the maximum possible long term carbon gains for the forest and its products, could be as little as 30 years.

Restoration

For information on the restoration of peatlands, read:

This article says “Bogs, swamps and mires help keep 500 billion metric tons of carbon out of the atmosphere, so preserving peatlands is emerging as a new priority.”

In the UK

It is reasonable to assume that the majority of peatbog erosion in the UK results from human action and thus warrants restoration. Where bogs have lost their soft protective top layer, as have a large proportion of UK peat bog, their carbon stores are being lost.

From Peat bogs and carbon, The rural information network.

References

Although peatlands cover only 3% of the Earth’s land surface, boreal and subarctic peatlands store about 15?30% 15–30% of the world’s soil carbon as peat. Despite their potential for large positive feedbacks to the climate system through sequestration and emission of greenhouse gases, peatlands are not explicitly included in global climate models and therefore in predictions of future climate change. In April 2007 a symposium was held in Wageningen, the Netherlands, to advance our understanding of peatland C cycling through integration across disciplines and research approaches and to develop a more synthetic picture of the present and future role of peatlands in the global C cycle and their interactions with the climate system. This paper aims to synthesize the main findings of the symposium, focusing on (i) small-scale processes, (ii) C fluxes at the landscape scale, and (iii) peatlands and climate. The paper concludes with a summary of the main drivers of the C balance of peatlands, and proposes directions for new research to reduce key uncertainties in our knowledge of C cycling in peatlands in order to facilitate the explicit inclusion of these ecosystems in a new generation of earth system models.

See also Wetland

category: ecology, carbon