Peat

Peat is an organic deposit formed on Earth's surface under waterlogged conditions. In most soils, dead organic matter is consumed in the presence of oxygen by organisms such as invertebrates, fungi, and bacteria about as quickly as it is added through processes such as leaf fall and plant death. The organic matter is oxidized to carbon dioxide gas and water, and over the long term, the amount of organic material in the soil hardly changes. However, if the sediments are waterlogged, oxygen becomes scarce because gases diffuse much more slowly in water than in air and the oxygen that is consumed cannot quickly be replaced. In the absence of oxygen, decomposition is slower and incomplete. This leads to the buildup of peat. Peat thus consists of partially decomposed remains of plants (and, in much less abundance, animals) that previously lived at the site of deposition. It may also contain small amounts of material such as wind-blown dust and chemical precipitates. (Most authors draw a distinction between in situ peats and organic lake sediments containing plant material that may have originated at some distance from the location of the deposits.)

Peat deposits are found in all but the most arid environments. Tropical peats tend to occur in basins, river deltas, sheltered coastal mangrove swamps, or estuaries—anywhere where water flows slowly enough to allow stagnation and anoxia to develop. At higher latitudes, peats frequently grow above the regional water table because certain peat-forming plants, chiefly Sphagnum, are able to retain water like a sponge; such peatlands are called raised bogs. At very high latitudes, the low temperatures and the presence of seasonal snow and ice can aid waterlogging in reducing decomposition rates.

Peatlands are ecologically important. Peatlands fed by groundwater are often very productive and can have high biodiversity. In contrast, raised bogs, being fed only by rainwater, tend to be acidic and poor in nutrients, with a highly specialized flora and fauna. As peat accumulates, it effectively records its own history in the form of layer upon layer of subfossilized plant and animal remains. Ecologists are able to use this information to [Page 2143]reconstruct the long-term development of these ecosystems.

Men cutting peat from a bog in Maamturk Mountains near Cong, Ireland

Source: Phil Augustavo/iStockphoto.

Peatlands are estimated to cover more than 4 million square kilometers, or 3%, of the land surface of Earth and contain approximately one third of all carbon stored in soils. Peatland destruction may therefore contribute to the increasing concentration of carbon dioxide in the atmosphere and hence to global warming. However, peatlands also produce methane as a byproduct of the decomposition processes. Methane is a much more powerful greenhouse gas than carbon dioxide, so the net effect of peatland loss on the climate system remains unclear. Climate change may itself upset the carbon balance of high-latitude peatlands, for example, by causing drying out and increased oxidation of organic matter, so peatlands may turn out to act as a positive feedback within the climate system.

The major cause of destruction of peatlands in the recent past has been drainage for forestry and agriculture; about 10% to 20% of the world's peatlands have been destroyed since AD 1800. Peat also has a range of commercial uses, notably as a fuel for heat and electricity production and in horticulture, and it is widely extracted for these purposes.

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