Editing Fen (section)

=== Nutrient cycling ===
Fen, being a distinct type of wetland, shares many [[Biogeochemistry|biogeochemical]] characteristics with other wetlands.<ref name=":0">{{Cite book|last=Mitsch|first=William J. |title=Wetlands |date=2007 |publisher=Wiley |author2=James G. Gosselink |isbn=978-0-471-69967-5 |edition=4th |location=Hoboken, NJ |oclc=78893363}}</ref> Like all wetlands, they play an important role in [[nutrient cycling]] because they are located at the interface of aerobic (oxic) and anaerobic (anoxic) environments.<ref name=":23"/> Most wetlands have a thin top layer of oxygenated soil in contact with the atmosphere or oxygenated surface waters.<ref name=":23" /> Nutrients and minerals may cycle between this oxidized top layer and the reduced layer below, undergoing oxidation and reduction reactions by the microbial communities adapted to each layer.<ref name=":0" /> Many important reactions take place in the reduced layer, including [[denitrification]], manganese reduction, iron reduction, sulfate reduction, and [[methanogenesis]].<ref name=":0" /> Because wetlands are hotspots for nutrient transformations and often serve as nutrient sinks, they may be constructed to treat nutrient-rich waters created by human activities.<ref name=":23" />

Fens are also hotspots for [[primary production]], as the continuous input of groundwater stimulates production.<ref name=":0" /> [[Bog]]s, which lack this input of [[groundwater]], have much lower primary production.<ref name=":0" />

==== Carbon ====
Carbon from all types of wetlands, including fens, arrives mostly as [[Organic matter|organic carbon]] from either adjacent upland ecosystems or by photosynthesis in the wetland itself.<ref name=":23" /> Once in the wetland, organic carbon generally has three main fates: oxidation to CO<sub>2</sub> by [[aerobic respiration]], burial as organic matter in peat, or decomposition to [[methane]].<ref name=":23" /> In peatlands, including fens, primary production by plants is greater than decomposition, which results in the accumulation of organic matter as peat. Resident mosses usually carry out decomposition within the fen, and temperate fens are often driven by plant roots' decomposition.<ref>{{Cite journal|last1=Scheffer|first1=Robbert A.|last2=Aerts|first2=Rien|date=December 2000|title=Root decomposition and soil nutrient and carbon cycling in two temperate fen ecosystems|url=http://dx.doi.org/10.1034/j.1600-0706.2000.910316.x|journal=Oikos|volume=91|issue=3|pages=541–549|doi=10.1034/j.1600-0706.2000.910316.x|bibcode=2000Oikos..91..541S |issn=0030-1299}}</ref> These peat stores sequester an enormous amount of carbon.<ref name=":0" /> Nevertheless, it is difficult to determine whether fens net take up or emit [[greenhouse gas]]es.<ref name=":16">{{Cite journal|last1=Loisel|first1=Julie|last2=van Bellen|first2=Simon|last3=Pelletier|first3=Luc|last4=Talbot|first4=Julie|last5=Hugelius|first5=Gustaf|last6=Karran|first6=Daniel|last7=Yu|first7=Zicheng|last8=Nichols|first8=Jonathan|last9=Holmquist|first9=James|date=2017-02-01|title=Insights and issues with estimating northern peatland carbon stocks and fluxes since the Last Glacial Maximum |journal=Earth-Science Reviews|language=en|volume=165|pages=59–80|doi=10.1016/j.earscirev.2016.12.001|bibcode=2017ESRv..165...59L|issn=0012-8252}}</ref> This is because fens emit methane, which is a more potent greenhouse gas than carbon dioxide.<ref name=":0" /> Methanogenic [[archaea]] that reside in the anaerobic layers of peat combine carbon dioxide and [[Hydrogen|hydrogen gas]] to form methane and water.<ref name=":23" /> This methane can then escape into the atmosphere and exert its warming effects.<ref name=":63">{{Cite book|last=Rydin|first=Håkan |title=The biology of peatlands |date=2013 |author2=J. K. Jeglum |isbn=978-0-19-150828-8 |edition=Second |location=Oxford, UK |oclc=861559248}}</ref> Peatlands dominated by brown mosses and sedges such as fens have been found to emit a greater amount of methane than ''[[Sphagnum]]''-dominated peatlands such as bogs.<ref name=":0" /><ref name=":16" />

==== Nitrogen ====
Fens play an important role in the global [[nitrogen cycle]] due to the anaerobic conditions found in their soils, which facilitate the oxidation or reduction of one form of nitrogen to another.<ref name=":23" /> Most nitrogen arrives in wetlands as nitrate from [[Surface runoff|runoff]], in organic matter from other areas, or by [[nitrogen fixation]] in the wetland.<ref name=":23" /> There are three main forms of nitrogen found in wetlands: nitrogen in organic matter, oxidized nitrogen ([[nitrate]] or [[nitrite]]), and [[ammonium]].<ref name=":63" />

Nitrogen is abundant in peat.<ref name=":63" /> When the organic matter in peat is decomposed in the absence of oxygen, ammonium is produced via [[ammonification]].<ref name=":23" /> In the oxidized surface layer of the wetland, this ammonium is oxidized to nitrite and nitrate by [[nitrification]].<ref name=":23" /> The production of ammonium in the reduced layer and its consumption in the top oxidized layer drives upward [[diffusion]] of ammonium.<ref name=":23" /> Likewise, nitrate production in the oxidized layer and nitrate consumption in the reduced layer by denitrification drives downward diffusion of nitrate.<ref name=":23" /> [[Denitrification]] in the reduced layer produces nitrogen gas and some [[nitrous oxide]], which then exit the wetland to the atmosphere.<ref name=":23" /> Nitrous oxide is a potent greenhouse gas whose production is limited by nitrate and nitrite concentrations in fens.<ref>{{Cite journal|last1=Palmer|first1=Katharina|last2=Horn|first2=Marcus A.|date=2015-04-10|title=Denitrification Activity of a Remarkably Diverse Fen Denitrifier Community in Finnish Lapland Is N-Oxide Limited |journal=PLOS ONE|language=en|volume=10|issue=4|pages=e0123123|doi=10.1371/journal.pone.0123123 |doi-access=free|issn=1932-6203|pmc=4393310|pmid=25860353|bibcode=2015PLoSO..1023123P}}</ref>

Nitrogen, along with phosphorus, controls how fertile a wetland is.<ref name=":23" />

==== Phosphorus ====
Almost all of the phosphorus that arrives in a wetland does so through sediments or plant litter from other ecosystems.<ref name=":23" /> Along with nitrogen, phosphorus limits wetland fertility.<ref name=":23" /> Under basic conditions like those found in extremely rich fens, calcium will bind to [[phosphate]] anions to make [[calcium phosphate]]s, which are unavailable for uptake by plants.<ref name=":23" /> Mosses also play a considerable role in aiding plants in phosphorus uptake by decreasing soil phosphorus stress and stimulating [[phosphatase]] activity in organisms found below the moss cover.<ref name=":5">{{Cite journal|last1=Crowley|first1=Katherine F.|last2=Bedford|first2=Barbara L.|date=September 2011|title=Mosses influence phosphorus cycling in rich fens by driving redox conditions in shallow soils|url=http://link.springer.com/10.1007/s00442-011-1970-8|journal=Oecologia|language=en|volume=167|issue=1|pages=253–264|doi=10.1007/s00442-011-1970-8|pmid=21445686|bibcode=2011Oecol.167..253C|s2cid=24302679|issn=0029-8549|access-date=2021-04-14|archive-date=2022-01-12|archive-url=https://web.archive.org/web/20220112234330/https://link.springer.com/article/10.1007%2Fs00442-011-1970-8|url-status=live}}</ref> Helophytes have been shown to bolster phosphorus cycling within fens, especially in fen reestablishment, due to their ability to act as a phosphorus sink, which prevents residual phosphorus in the fen from being transferred away from the it.<ref>{{Cite journal|last1=Zak|first1=Dominik|last2=Gelbrecht|first2=Jörg|last3=Zerbe|first3=Stefan|last4=Shatwell|first4=Tom|last5=Barth|first5=Martin|last6=Cabezas|first6=Alvaro|last7=Steffenhagen|first7=Peggy|date=May 2014|title=How helophytes influence the phosphorus cycle in degraded inundated peat soils – Implications for fen restoration|url=https://linkinghub.elsevier.com/retrieve/pii/S0925857413004187|journal=Ecological Engineering|language=en|volume=66|pages=82–90|doi=10.1016/j.ecoleng.2013.10.003|bibcode=2014EcEng..66...82Z |access-date=2021-04-14|archive-date=2018-07-01|archive-url=https://web.archive.org/web/20180701165754/https://linkinghub.elsevier.com/retrieve/pii/S0925857413004187|url-status=live}}</ref> Under normal conditions, phosphorus is held within soil as dissolved inorganic phosphorus, or [[phosphate]], which leaves trace amounts of phosphorus in the rest of the ecosystem.<ref>{{Cite journal|last1=Richardson|first1=Curtis J.|last2=Marshall|first2=Paul E.|date=December 1986|title=Processes Controlling Movement, Storage, and Export of Phosphorus in a Fen Peatland|url=https://onlinelibrary.wiley.com/doi/10.2307/1942548|journal=Ecological Monographs|language=en|volume=56|issue=4|pages=279–302|doi=10.2307/1942548|jstor=1942548|bibcode=1986EcoM...56..279R |issn=0012-9615}}</ref>

Iron is important in phosphorus cycling within fens. Iron can bind to high levels of inorganic phosphate within the fen, leading to a toxic environment and inhibition of plant growth.<ref name=":5" /> In iron-rich fens, the area can become vulnerable to acidification, excess nitrogen and potassium, and low water levels.<ref name=":8">{{Cite journal|last1=Kooijman|first1=A. M.|last2=Cusell|first2=C.|last3=Hedenäs|first3=L.|last4=Lamers|first4=L. P. M.|last5=Mettrop|first5=I. S.|last6=Neijmeijer|first6=T.|date=February 2020|title=Re-assessment of phosphorus availability in fens with varying contents of iron and calcium|journal=Plant and Soil|language=en|volume=447|issue=1–2|pages=219–239|doi=10.1007/s11104-019-04241-4|s2cid=208649335|issn=0032-079X|doi-access=free|bibcode=2020PlSoi.447..219K |hdl=2066/214408|hdl-access=free}}</ref> Peat soils play a role in preventing the bonding of irons to phosphate by providing high levels of organic anions for iron to bind to instead of inorganic anions such as phosphate.<ref name=":8" />