Wetland Geomorphology and Soils Chapter 2

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Wetland Geomorphology and Soils Chapter 2
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Biogeochemistry

• interrelated physical, chemical, biological
  processes
• soil is where these interactions occur

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Oxidizing Conditions

• losing electrons
• first used to describe oxygen combining
  chemically with another substance
• now includes other reactions
• oxidation of one compound involves reduction of
  another

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Reducing Conditions

• decrease in oxidation state
• gaining electrons
• more common before evolution of photosynthetic
  organisms
• now represented by a smaller part of Earths
  surface namely wetlands

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Earths Atmospheric Changes

• splitting of water molecules during
  photosynthesis
• amount of oxygen gas increased in the atmosphere

Biotic feedback
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Hydric Soils

• form when oxygen is cut off
• reduced conditions dominate
• medium in which chemical transformations take
  place

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Organic Soils

• Page 13
• Organic Carbon content gt 12-18
• Saturated
• No clay 12 organic Carbon
• 60 clay 18 organic Carbon
• Not saturated
• Has 20 organic Carbon

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Mineral Soils

• lt 20-35 OM
• if not organic, considered mineral
• does not mean that it cannot have an upper
  organic peat layer (e.g., swamp forests may have
  a 25 cm OM layer)

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Mineral Soils

• lt 20-35 OM
• if not organic, considered mineral
• does not mean that it cannot have an upper
  organic peat layer (e.g., swamp forests may have
  a 25 cm OM layer)

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Organic vs. Mineral Soils

• Organic soils
• higher porosity
• reduced bulk density (dry weight/known volume)
• higher hydraulic conductivity (ability to move
  water)
• lower nutrient availability because tied up in
  organic form
• higher cation exchange capacity

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Cation Exchange Capacity Organic Soils

• sum of exchangeable cations (positive ions) is
  higher
• H, Ca 2, Mg 2, K, and Na

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Cation Exchange CapacityMineral Soils

• can hold less cations
• mineral soils dominated by the major metal cations

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Organic Content affects exchangeable ions

• with increased organic content, Hydrogen ions
  increase in and amount
• metal ions have a hump-shaped response

exchangeable ions
organic content
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Composition and Origin of an Organic Soil

• mostly undecomposed remains of plants
• builds up because decomposition is slow under
  anaerobic conditions
• botanical origins
• mosses Sphagnum spp.
• herbaceous Panicum spp., Carex spp.
• woody material
• plant remains
• degree to which it is decomposed determines CEC,
  porosity, conductivity

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Classification of Organic Soils based upon
Decomposition

• Fibrists (peat) lt 1/3 decomposed
• Hemists (mostly peat) intermediate
• Saprists (muck) 2/3 decomposed

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Mineral Soils

• when flooded, develop redoximorphic features
• refers to reduction/oxidation of iron and
  manganese oxides
• mediated by microbial activity
• conditions required
• anaerobic
• above 5 C
• have organic material as a substrate for microbes

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Redox Depletions

• when soil is flooded for sufficient amount of
  time, get grey to greenish blue colorations
• due to reduction of iron to a soluble form Fe 2
  (and Mn 2)
• if leached from soil, leaves natural color of
  parent material
• reduced and then depleted from matrix
• development of darker color gleying
• may be at small scale
• or entire soil matrix may become gleyed

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Redox Concentrations

• if not flooded, have a brownish, reddish color
• would tell you that drying occurs
• may have mottles surrounded by gleyed matrix
• dominance of iron oxides Fe 3
• manganese oxides black color Mn 3 or Mn 4

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Oxidized Rhizosphere

• some hydrophytic species can pump oxygen down
  their roots
• will see evidence of iron oxides and manganese
  oxides even after death of plant

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Munsell Chart

• standardized color chart for comparing soil
  color
• used a lot to delineate wetlands
• hydric soils usually have a 2 or less in chroma
  on the chart