Peatland

A peatland is a type of wetland whose soils consist of organic matter from decaying plants, forming layers of peat. Peatlands arise because of incomplete decomposition of organic matter, usually litter from vegetation, due to water-logging and subsequent anoxia. Peatlands are unusual landforms that derive mostly from biological rather than physical processes, and can take on characteristic shapes and surface patterning.
The formation of peatlands is primarily controlled by climatic conditions such as precipitation and temperature, although terrain relief is a major factor as waterlogging occurs more easily on flatter ground and in basins. Peat formation typically initiates as a paludification of a mineral soil forest, terrestrialisation of lakes, or primary peat formation on bare soils on previously glaciated areas. A peatland that is actively forming peat is called a mire. All types of mires share the common characteristic of being saturated with water, at least seasonally with actively forming peat, while having their own ecosystem.
Peatlands are the largest natural carbon store on land. Covering around 3 million km² globally, they sequester 0.37 gigatons of carbon dioxide a year. Peat soils store over 600 Gt of carbon, more than the carbon stored in all other vegetation types, including forests. In their natural state, peatlands provide a range of ecosystem services, including minimising flood risk and erosion, purifying water and regulating climate.
Peatlands are under threat by commercial peat harvesting, drainage and conversion for agriculture and fires, which are predicted to become more frequent with climate change. The destruction of peatlands results in release of stored greenhouse gases into the atmosphere, further exacerbating climate change.

Types

For botanists and ecologists, the term peatland is a general term for any terrain dominated by peat to a depth of at least, even if it has been completely drained. A peatland that is still capable of forming new peat is called a mire, while drained and converted peatlands might still have a peat layer but are not considered mires as the formation of new peat has ceased.
There are two types of mire: bog and fen. A bog is a mire that, due to its raised location relative to the surrounding landscape, obtains all its water solely from precipitation. A fen is located on a slope, flat, or in a depression and gets most of its water from the surrounding mineral soil or from groundwater. Thus, while a bog is always acidic and nutrient-poor, a fen may be slightly acidic, neutral, or alkaline, and either nutrient-poor or nutrient-rich. All mires are initially fens when the peat starts to form, and may turn into bogs once the height of the peat layer reaches above the surrounding land. A quagmire is a floating mire, bog, or any peatland being in a stage of hydrosere or hydrarch succession, resulting in pond-filling yields underfoot. Ombrotrophic types of quagmire may be called quaking bog. Minerotrophic types can be named with the term quagfen.
Some swamps can also be peatlands, while marshes are generally not considered to be peatlands. Swamps are characterized by their forest canopy or the presence of other tall and dense vegetation like papyrus. Like fens, swamps are typically of higher pH level and nutrient availability than bogs. Some bogs and fens can support limited shrub or tree growth on hummocks. A marsh is a type of wetland within which vegetation is rooted in mineral soil.

Global distribution

Peatlands are found around the globe, although are at their greatest extent at high latitudes in the Northern Hemisphere. Peatlands are estimated to cover around 3% of the globe's surface, although estimating the extent of their cover worldwide is difficult due to the varying accuracy and methodologies of land surveys from many countries. Mires occur wherever conditions are right for peat accumulation: largely where organic matter is constantly waterlogged. Hence the distribution of mires is dependent on topography, climate, parent material, biota and time. The type of mire—bog, fen, marsh or swamp—depends also on each of these factors.
The largest accumulation of mires constitutes around 64% of global peatlands and is found in the temperate, boreal and subarctic zones of the Northern Hemisphere. Mires are usually shallow in polar regions because of the slow rate of accumulation of dead organic matter, and often contain permafrost and palsas. Very large swathes of Canada, northern Europe and northern Russia are covered by boreal mires. In temperate zones mires are typically more scattered due to historical drainage and peat extraction, but can cover large areas. One example is blanket bog where precipitation is very high i.e., in maritime climates inland near the coasts of the north-east and south Pacific, and the north-west and north-east Atlantic. In the sub-tropics, mires are rare and restricted to the wettest areas.
Mires can be extensive in the tropics, typically underlying tropical rainforest. Tropical peat formation is known to occur in coastal mangroves as well as in areas of high altitude. Tropical mires largely form where high precipitation is combined with poor conditions for drainage. Tropical mires account for around 11% of peatlands globally, and are most commonly found at low altitudes, although they can also be found in mountainous regions, for example in South America, Africa and Papua New Guinea. Indonesia, particularly on the islands of Sumatra, Kalimantan and Papua, has one of the largest peatlands in the world, with an area of about 24 million hectares. These peatlands play an important role in global carbon storage and have very high biodiversity. However, peatlands in Indonesia also face major threats from deforestation and forest fires. In the early 21st century, the world's largest tropical mire was found in the Central Congo Basin, covering 145,500 km² and storing up to 10¹³ kg of carbon.
The total area of mires has declined globally due to drainage for agriculture, forestry and peat harvesting. For example, more than 50% of the original European mire area which is more than 300,000 km² has been lost. Some of the largest losses have been in Russia, Finland, the Netherlands, the United Kingdom, Poland and Belarus. A catalog of the peat research collection at the University of Minnesota Duluth provides references to research on worldwide peat and peatlands.

Biochemical processes

Peatlands have unusual chemistry that influences, among other things, their biota and water outflow. Peat has very high cation-exchange capacity due to its high organic matter content: cations such as Ca²⁺ are preferentially adsorbed onto the peat in exchange for H⁺ ions. Water passing through peat declines in nutrients and pH. Therefore, mires are typically nutrient-poor and acidic unless the inflow of groundwater is high.
Generally, whenever the inputs of carbon into the soil from dead organic matter exceed the carbon outputs via organic matter decomposition, peat is formed. This occurs due to the anoxic state of water-logged peat, which slows down decomposition. Peat-forming vegetation is typically also recalcitrant due to high lignin and low nutrient content. Topographically, accumulating peat elevates the ground surface above the original topography. Mires can reach considerable heights above the underlying mineral soil or bedrock: peat depths of above 10 m have been commonly recorded in temperate regions, and above 25 m in tropical regions.^{Mire#cite note-Rydin2006-7|} When the absolute decay rate of peat in the catotelm matches the rate of input of new peat into the catotelm, the mire will stop growing in height.^{Mire#cite note-8|}

Carbon storage and methanogenesis

Despite accounting for just 3% of Earth's land surfaces, peatlands are collectively a major carbon store containing between 500 and 700 billion tonnes of carbon. Carbon stored within peatlands equates to over half the amount of carbon found in the atmosphere. Peatlands interact with the atmosphere primarily through the exchange of carbon dioxide, methane and nitrous oxide, and can be damaged by excess nitrogen from agriculture or rainwater. The sequestration of carbon dioxide takes place at the surface via the process of photosynthesis, while losses of carbon dioxide occur through living plants via autotrophic respiration and from the litter and peat via heterotrophic respiration. In their natural state, mires are a small atmospheric carbon dioxide sink through the photosynthesis of peat vegetation, which outweighs their release of greenhouse gases. On the other hand, most mires are generally net emitters of methane and nitrous oxide. Due to the continued sequestration over millennia, and because of the longer atmospheric lifespan of the molecules compared with methane and nitrous oxide, peatlands have had a net cooling effect on the atmosphere.
The water table position of a peatland is the main control of its carbon release to the atmosphere. When the water table rises after a rainstorm, the peat and its microbes are submerged under water inhibiting access to oxygen, reducing release via respiration. Carbon dioxide release increases when the water table falls lower, such as during a drought, as this increases the availability of oxygen to the aerobic microbes thus accelerating peat decomposition. Levels of methane emissions also vary with the water table position and temperature. A water table near the peat surface gives the opportunity for anaerobic microorganisms to flourish.
Methanogens are strictly anaerobic organisms and produce methane from organic matter in anoxic conditions below the water table level, while some of that methane is oxidised by methanotrophs above the water table level. Therefore, changes in water table level influence the size of these methane production and consumption zones. Increased soil temperatures also contribute to increased seasonal methane flux. A study in Alaska found that methane may vary by as much as 300% seasonally with wetter and warmer soil conditions due to climate change.
Peatlands are important for studying past climate because they are sensitive to changes in the environment and can reveal levels of isotopes, pollutants, macrofossils, metals from the atmosphere and pollen. For example, carbon-14 dating can reveal the age of the peat. The dredging and destruction of a peatland will release the carbon dioxide that could reveal irreplaceable information about the past climatic conditions. Many kinds of microorganisms inhabit peatlands, due to the regular supply of water and abundance of peat forming vegetation. These microorganisms include but are not limited to methanogens, algae, bacteria, zoobenthos, of which Sphagnum species are most abundant.