peatlands

A mire, peatland, or quagmire is a wetland area dominated by living peat-forming plants. Mires arise because of incomplete decomposition of organic matter, usually litter from vegetation, due to water-logging and subsequent anoxia. 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. Like coral reefs, mires are unusual landforms that derive mostly from biological rather than physical processes, and can take on characteristic shapes and surface patterning.

A quagmire is a floating (quaking) mire, bog, or any peatland being in a stage of hydrosere or hydrarch (hydroseral) succession, resulting in pond-filling yields underfoot. Ombrotrophic types of quagmire may be called quaking bog (quivering bog). Minerotrophic types can be named with the term quagfen.

There are four types of mire: bog, fen, marsh and swamp. A bog is a mire that, due to its location relative to the surrounding landscape, obtains most of its water from rainfall ( ombrotrophic). A fen is located on a slope, flat, or in a depression and gets most of its water from soil or groundwater (minerotrophic). 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. A marsh is a type of wetland within which vegetation is rooted in mineral soil but some marshes form shallow peat deposits well known as Mires. Swamps are characterized by their forest canopy and, like fens, are typically of higher pH level and nutrient availability than bogs. Some bogs and fens can support limited shrub or tree growth on hummocks.

The formation of mires today 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. However, there is a growing anthropogenic influence in the accumulation of peat and peatlands around the world, including through both conservation efforts as well as climate change-induced destruction by droughts and forest fires. Topographically, mires elevate the ground surface above the original topography. Mires can reach considerable heights above the underlying mineral soil or bedrock: peat depths of above 10m have been commonly recorded in temperate regions (many temperate and most boreal mires were removed by ice sheets in the last Ice Age), and above 25 m in tropical regions. ^{/sup> When the absolute decay rate in the ''catotelm'' (the lower, water-saturated zone of a mire) matches the rate of input of new peat into the catotelm, the mire will stop growing in height. /sup> A simplistic calculation, using typical values for a Sphagnum bog of 1mm new peat added per year and 0.0001 proportion of the catotelm decaying per year, gives a maximum height of 10 m. More advanced analyses incorporate expectable nonlinear rates of catotelm decay.}

For botanists and ecologists, the term peatland is a more general term for any terrain dominated by peat to a depth of at least , even if it has been completely drained (i.e., a peatland can be dry, but a mire by definition must be actively forming peat).

Global distribution

Mires, although at their greatest extent at high latitudes in the Northern Hemisphere, are found around the globe. Estimating the extent of mire land 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 accumulations of mires constitute of around 64% of global peatlands and are 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 is often known to contain permafrost. Very large swathes of Canada, northern Europe and northern Russia are covered by boreal mires. In temperate areas 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 (for example, in Kalimantan). 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 (more than half of which can be found in Southeast Asia), 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. Recently, the world's largest tropical mire was found in the Central Congo Basin, covering 145,500 square kilometres and it may store up to 10¹³ kg of carbon.

Mires have declined globally due to drainage for agriculture, forestry, and for peat harvesting. For example, more than 50% of original European mire area which is more than 300,000 square kilometers has been lost. Some of the largest losses have been in Russia, Finland, the Netherlands, the United Kingdom, Poland and Belarus.

Bio-chemical processes

Mires have unusual chemistry that influences, inter alia, 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 in pH. Therefore, mires are typically nutrient-poor and acidic unless the inflow of groundwater (bringing in supplementary cations) is high.

Generally, whenever the inputs of carbon exceed carbon outputs, mires are formed. This occurs due to the anoxic state of water-logged peat, and the process of photosynthesis by which peat grows. Despite accounting for just 3% of Earth's land surfaces, mires are collectively a major carbon store containing between 500 and 700 billion tonnes of carbon. Carbon stored within mires equates to over half the amount of carbon found in the atmosphere. Mires 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 peat tissue via respiration. In their natural state, mires are a slight atmospheric carbon dioxide sink through the photosynthesis of peat vegetation, which outweighs their release of greenhouse gases. In addition, most mires are generally net emitters of methane and nitrous oxide.

The water table position of a mire is responsible for its carbon release to the atmosphere. When the water table rises post rainstorm, the peat and its microbes are submerged under water inhibiting access to oxygen, reducing respiration, and releasing carbon dioxide. Carbon dioxide release increases when the water table shrinks, such as during a drought, as this supplies the aerobic microbes with oxygen to decompose the peat. Levels of methane also vary with the water table position and somewhat with temperature. A water table near the peat surface gives the opportunity for anaerobic microorganisms to flourish. Methanogens are responsible for producing methane via decomposition of the peat which consequently increases as the water table rises and oxygen levels are depleted. Increased temperatures in the soil also contributes to increased seasonal methane flux, though at a lower intensity. It is shown that the methane increased by as much as 300% seasonal from increased precipitation and temperature of the soil.

Mires are important reservoirs of climatic information to the past 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 mire will release the carbon dioxide that could reveal irreplaceable information about the past climatic conditions. It is widely known that a plethora of microorganisms inhabit mires 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

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