Wetland biogeochemistry Chapter 6 Mitsch

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Wetland biogeochemistryChapter 6Mitsch
Gosselink
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Basic definitions information

• biogeochemical cycling transport
  transformation of chemicals in ecosystems
• intrasystem cycling transformation processes
• exchange of chemicals between wetlands
  surrounding habitats
• sinks vs. sources

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Organic soils

• more minerals tied up in forms unavailable to
  plants
• higher organic content
• higher organic carbon
• greater cation exchange capacity

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Oxidizing agent accepts electrons Reducing
agent donates electrons
e.g. oxidation of Fe 2 ? Fe 3
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Redox potential

• Measure of electron (e-) pressure or availability
  in a solution
• Concentrations of oxidants and reductants in
  soils influences tendency to oxidize or reduce
• Drops as substances are oxidized
• Organic substances (e- donors) oxidized with
  O2, NO3-, Mn2 ,
  Fe2 , SO42-

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Order of oxidation is determined by redox
potential highest ? lowest
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Order of oxidation / reduction reactions in
sediments

• Aerobic oxidation O2 reduced
  ? H2O
• N reduction NO3- ? NO2- ? NO ? N2O
  ? N2
• Manganese reduction Mn4 ? Mn2
• Iron reduction Fe2
  ? Fe3
• Sulfate reduction SO42- ?
  S2-
• CO2 reduction CO2 ? CH4

• Oxidized forms are reduced in order of redox
  potential

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Exercise in Nitrogen cycling

• sources and sinks of N in wetlands
• which forms are are not bioavailable?
• which forms are mobile in wetland sediments, and
  which are not (or less mobile)
• Figure out what happens in each process
• N-fixation
• nitrification
• denitrification
• ammonification

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Nitrogen cycling (N-fixation)

• atmospheric N2 ? organically-bound N
• aerobic anaerobic
• conducted by cyanobacteria in soils or N-nodule
  bacteria in rhizosphere
• location of reactions both surface and
  subsurface soils

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Nitrogen cycling (nitrification)

• NH3 ? NO2- ? NO3-
• Exclusively aerobic
• Conducted by bacteria
• NH4 ? NO2- (Nitrosomonas sp.)
• NO2- ? NO3- (Nitrobacter sp.)

Root nodules on clover
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Nitrogen cycling (denitrification)

• NO3- ? NO2- ? NO ? NO2 ? N2 (N2 lost to
  atmosphere)
• Strictly anaerobic conditions
• Waterlogged aerobic soils
• Returns N2 to the atmosphere
• Significant pathway of N loss from wetlands

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Nitrogen cycling (ammonification)

• R-NH2 ? NH3
• (R-NH2 organically-bound NH2)
• NH3 reacts with water to form NH4
• NH4 is easily absorbed by plant roots
• NH4 is the primary form of N in wetland soils

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Nitrogen cycling
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Nitrogen cycling
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Iron transformation

• Primarily found in reduced form (Fe2) in
  wetlands
• oxidation to Fe3 by aerobic chemosynthetic
  bacteria
• can reach toxic concentrations in sediments
• Fe3 precipitates at surface (bog iron), gives
  sediment red/brown color
• Fe2 gives sediment greenish/grey color
• FeS gives sediment the black color of anaerobic
  soils

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Sulfur transformation

• S is rarely limiting in wetlands
• S reduction by obligate anaerobes (e.g.
  Desulfovibrio)
• produces H2S, volatilized into atmosphere
• S oxidation by chemoautotrophic photosynthetic
  microorganisms (back to SO4)
• H2S can be toxic to plant roots, microbes,
  animals
• Sources atmosphere, runoff (SO4)
• Sinks loss to atmosphere (H2S, DMS)

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Carbon transformation

• In aerobic sediments photosynthesis,
  respiration
• - CO2, organic matter (e.g. sugars)
• In anaerobic sediments fermentation
  (glycolysis)
• - by anaerobic microorganisms- produces low
  molecular wt. acids, alcohols, CO2
• Methanogenesis methanogens (Archaea) use CO2
  and produce CH4 swamp gas or marsh gas
• Sources CO2 (atmosphere), runoff, precipitation
• Sinks CH4, CO2 gases lost to atmosphere

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Phosphorous transformation

• important limiting nutrient, esp. in FW wetlands
• not limiting in salt marshes
• bioavailable forms are soluble, inorganic
• (e.g. H2PO4-, HPO42-, PO43-)
• unavailable P in organic litter, peat, sediments
  (e.g. clay adsorption), complexed to Ca, Fe, Al,
  or bound in living tissue (plants, animals)
• P cycling tied to sedimentation resuspension

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Wetland biogeochemistry - overall

• wetlands can be sources, sinks, or transformers
• seasonal patterns of nutrient uptake / release
• coupling to adjacent ecosystems (up- /
  downstream)
• can have high or low productivity
• nutrients tied up in sediments peat
• (vs. aquatic, terrestrial systems)
• long-term anthropogenic impacts on nutrient
  cycling are unclear / unknown

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Questions

• 1. What is the order of reduction in wetland
  sediments?
• i.e. whats reduced first, second, etc.??
• 2. How does the order of reduction play out
  relative to depth in the sediments?
• i.e. whats reduced at the surface, just below
  it, and so on as you go deeper?
• why does nutrient reduction change with depth?

Hint for 2 you might want to draw a graph