Editing Fen (section)

== Biogeochemical features ==
[[File:Spaulding Fen.jpg|thumb|upright=1.3|Spaulding Fen, Wisconsin.]]

=== Hydrological conditions ===
[[Hydrology|Hydrological]] conditions, as seen in other wetlands, are a major determinant of fen biota and [[biogeochemistry]].<ref name=":23">{{Cite book|last=Keddy|first=Paul A. |title=Wetland ecology: principles and conservation |date=2010 |publisher=Cambridge University Press |isbn=978-1-139-22365-2 |edition=2nd |location=Cambridge |oclc=801405617}}</ref> Fen soils are constantly inundated because the water table is at or near the surface.<ref name=":6">{{Cite book|last=Rydin|first=Håkan |title=The biology of peatlands |date=2013 |others=J. K. Jeglum |isbn=978-0-19-150828-8 |edition=Second |location=Oxford, UK |oclc=861559248}}</ref> The result is anaerobic (oxygen-free) soils due to the slow rate at which oxygen diffuses into waterlogged soil.<ref name=":23"/> Anaerobic soils are ecologically unique because Earth's atmosphere is oxygenated, while most terrestrial ecosystems and surface waters are aerobic. The anaerobic conditions found in wetland soils result in [[Redox|reduced]], rather than [[oxidized]], soil chemistry.<ref name=":23"/>

A hallmark of fens is that a significant portion of their water supply is derived from [[groundwater]] (minerotrophy).<ref name=":6" /> Because hydrology is the dominant factor in wetlands, the chemistry of the groundwater has an enormous effect on the characteristics of the fen it supplies.<ref name=":3">{{Cite journal |last1=Godwin |first1=Kevin S. |last2=Shallenberger |first2=James P. |last3=Leopold |first3=Donald J. |last4=Bedford |first4=Barbara L. |date=December 2002 |title=Linking landscape properties to local hydrogeologic gradients and plant species occurrence in minerotrophic fens of New York State, USA: A Hydrogeologic Setting (HGS) framework |journal=Wetlands |volume=22 |issue=4 |pages=722–737 |doi=10.1672/0277-5212(2002)022[0722:llptlh]2.0.co;2 |s2cid=20623975 |issn=0277-5212 |url=https://link.springer.com/article/10.1672/0277-5212%282002%29022%5B0722%3ALLPTLH%5D2.0.CO%3B2 |url-access=subscription |access-date=2021-04-05 |archive-date=2018-06-04 |archive-url=https://web.archive.org/web/20180604201340/https://link.springer.com/article/10.1672%2F0277-5212%282002%29022%5B0722%3ALLPTLH%5D2.0.CO%3B2 |url-status=live }}</ref> Groundwater chemistry, in turn, is largely determined by the geology of the rocks that the groundwater flows through.<ref>{{cite book |last=Fitts |first=Charles R. |chapter=10 – Groundwater Chemistry |date=2013 |doi=10.1016/B978-0-12-384705-8.00010-8 |title=Groundwater Science |edition=Second |pages=421–497 |editor-last=Fitts|editor-first=Charles R. |location=Boston |publisher=Academic Press |language=en |isbn=978-0-12-384705-8}}</ref> Thus, the characteristics of a fen, especially its pH, are directly influenced by the type of rocks its groundwater supply contacts. pH is a major factor in determining fen species composition and richness, with more basic fens called "rich" and more acidic fens called "poor."<ref name=":6" /> Rich fens tend to be highly biodiverse and harbor a number of rare or endangered species, and biodiversity tends to decrease as the richness of fen decreases.<ref name=":3" /><ref name=":6" />

Fens tend to be found above rocks that are rich in calcium, such as [[limestone]].<ref name=":23"/> When groundwater flows past calcareous (calcium-rich) rocks like limestone ([[calcium carbonate]]), a small amount dissolves and is carried to the fen supplied by the groundwater.<ref name=":12">{{Cite book|last=Clark|first=Ian|title=Environmental Geochemistry of Isotopes|publisher=Unpublished|year=2006|location=University of Ottawa|pages=1–7|chapter=Chapter 6: Weathering}}</ref> When calcium carbonate dissolves, it produces [[bicarbonate]] and a [[calcium]] [[cation]] according to the following equilibrium:<ref name=":12" />

<chem>CaCO3 + H2CO3 <=> Ca^2+ + 2HCO3^-</chem>

where [[carbonic acid]] (H<sub>2</sub>CO<sub>3</sub>) is produced by the dissolution of [[carbon dioxide]] in water.<ref name=":12" /> In fens, the bicarbonate anion produced in this equilibrium acts as a pH buffer, which keeps the pH of the fen relatively stable.<ref name=":11">{{Cite journal|last=Bourbonniere|first=Richard A.|date=January 2009 |title=Review of Water Chemistry Research in Natural and Disturbed Peatlands |journal=Canadian Water Resources Journal |language=en |volume=34 |issue=4 |pages=393–414 |doi=10.4296/cwrj3404393|bibcode=2009CaWRJ..34..393B |issn=0701-1784 |doi-access=free}}</ref> Fens supplied by groundwater that doesn't flow through minerals and act as a [[Buffer solution|buffer]] when dissolved tend to be more acidic.<ref name=":10">{{Cite journal|last1=Bedford|first1=Barbara L.|last2=Godwin|first2=Kevin S.|date=September 2003 |title=Fens of the United States: Distribution, characteristics, and scientific connection versus legal isolation |journal=Wetlands|volume=23|issue=3|pages=608–629|doi=10.1672/0277-5212(2003)023[0608:fotusd]2.0.co;2 |s2cid=24228048 |issn=0277-5212}}</ref> The same effect is observed when groundwater flows through minerals with low solubility, such as sand.<ref name=":10" />

In extreme rich fens, calcium carbonate can [[Precipitation (chemistry)|precipitate]] out of solution to form [[marl]] deposits.<ref name=":10" /> Calcium carbonate precipitates out of solution when the [[partial pressure]] of carbon dioxide in the solution falls.<ref name=":13">{{Cite journal|last=Bartigs|first=Rodney|date=March 1984|title=Marl Wetlands in Eastern West Virginia: Distribution, Rare Plant Species, and Recent History|journal=Castanea|volume=49|pages=17–25}}</ref> The decrease in carbon dioxide partial pressure is caused by uptake by plants for photosynthesis or direct loss to the atmosphere.<ref name=":13" /> This reduces the availability of carbonic acid in solution, [[Le Chatelier's principle|shifting the above equilibrium]] back towards the formation of calcium carbonate. The result is the precipitation of calcium carbonate and the formation of marl.<ref name=":13" />

=== 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" />

=== Bog–rich-fen gradient ===
Bogs and fens can be thought of as two ecosystems on a gradient from poor to rich, with bogs at the poor end, extremely rich fens at the rich end, and poor fens in between.<ref>{{Cite journal|last1=Szumigalski|first1=Anthony R.|last2=Bayley|first2=Suzanne E.|date=December 1996 |title=Net above-ground primary production along a bog-rich fen gradient in Central Alberta, Canada |journal=Wetlands|volume=16|issue=4|pages=467–476|doi=10.1007/bf03161336|bibcode=1996Wetl...16..467S |s2cid=24686070|issn=0277-5212}}</ref> In this context, "rich" and "poor" refer to the species richness, or how [[Biodiversity|biodiverse]] a fen or bog is.<ref name=":6" /> The richness of these species is strongly influenced by pH and concentrations of calcium and bicarbonate. These factors assist in identifying where along the gradient a particular fen falls.<ref>{{Cite journal|last=Bourbonniere|first=Richard A.|date=January 2009 |title=Review of Water Chemistry Research in Natural and Disturbed Peatlands |journal=Canadian Water Resources Journal|language=en|volume=34|issue=4|pages=393–414|doi=10.4296/cwrj3404393|bibcode=2009CaWRJ..34..393B |s2cid=98764979|issn=0701-1784}}</ref> In general, rich fens are [[minerotrophic]], or dependent on mineral-rich groundwater, while bogs are [[ombrotrophic]], or dependent on precipitation for water and nutrients.<ref name=":6" /> Poor fens fall between these two.

==== Rich fens ====
[[File:Extreme Rich Fen.jpg|thumb|upright=1.3|Small extreme rich fen in southwestern Minnesota. The white flowers, ''[[Parnassia glauca]]'', are a fen indicator species in Minnesota.]]
Rich fens are strongly minerotrophic; that is, a large proportion of their water comes from mineral-rich ground or surface water. Fens that are more distant from surface waters such as rivers and lakes, however, are more rich than fens that are connected.<ref name=":3" /> This water is dominated by calcium and bicarbonate, resulting in a slightly acidic to slightly basic pH characteristic of rich fens.<ref name=":6" /><ref name=":14">{{cite book |last1=Zoltai |first1=S. C. |last2=Vitt |first2=D. H. |chapter=Canadian wetlands: Environmental gradients and classification |date=1995 |doi=10.1007/978-94-011-0427-2_11 |editor=C. Max Finlayson |editor2=A. G. van der Valk |title=Classification and Inventory of the World's Wetlands |pages=131–137 |location=Dordrecht |publisher=Springer Netherlands |isbn=978-94-010-4190-4}}</ref> These conditions promote high biodiversity. Within rich fens, there is a large amount of variability. The richest fens are the extreme rich (marl) fens, where marl deposits are often build up.<ref name=":10" /> These are often pH 7 or greater.<ref name=":6" /> Rich and intermediate rich fens are generally neutral to slightly acidic, with a pH of approximately 7 to 5. Rich fens are not always very productive; at high calcium concentrations, calcium ions bind to phosphate anions, reducing the availability of phosphorus and decreasing primary production.<ref name=":23"/><ref name=":6" /> Rich bogs with limited primary production can stabilize with the accumulation of mosses and [[mycorrhiza]], which promote phosphorus cycling and can support the growth of new vegetation and bacteria.<ref name=":5" /> Brown mosses (family ''[[Amblystegiaceae]]'') and sedges (genus ''[[Carex]]'') are the dominant vegetation.<ref name=":14" /> However, an accumulation of mosses such as ''[[Sphagnum]]'' can lead to the acidification of the rich fen, potentially converting it into a poor fen.<ref>{{Cite web|title=Poor Fen - Michigan Natural Features Inventory|url=https://mnfi.anr.msu.edu/communities/description/10662/Poor-Fen|access-date=2021-05-08|website=mnfi.anr.msu.edu|archive-date=2021-05-08|archive-url=https://web.archive.org/web/20210508211654/https://mnfi.anr.msu.edu/communities/description/10662/Poor-Fen|url-status=live}}</ref> Compared to poor fens, rich fens have higher concentrations of bicarbonate, base cations (Na<sup>+</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>), and [[sulfate]].<ref name=":11" />

==== Poor fens ====
Poor fens are, in many ways, an intermediate between rich fens and bogs. Hydrologically, they are more similar to rich fens than to bogs, but regarding vegetation composition and chemistry, they are more similar to bogs than rich fens.<ref name=":14" /> They are much more acidic than their rich counterparts, with a pH of approximately 5.5 to 4.<ref name=":6" /> Peat in poor fens tends to be thicker than that of rich fens, which cuts off vegetation access to the mineral-rich soil underneath.<ref name=":23"/> In addition, the thicker peat reduces the influence of mineral-rich groundwater that buffers the pH.<ref name=":23"/> This makes the fen more ombrotrophic, or dependent on nutrient-poor precipitation for its water and nutrients.<ref name=":23"/> Poor fens may also form in areas where the groundwater supplying the fen flows through sediments that don't dissolve well or have low buffering capacity when dissolved.<ref name=":10" /> Species richness tends to be lower than that of rich fens but higher than that of bogs.<ref name=":6" /> Poor fens, like bogs, are dominated by ''Sphagnum'' mosses, which acidify the fen and decrease nutrient availability.<ref name=":14" />