nuap 2014 3. biogeochemistry
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Biogeochemistry
Dr. Paul J. DuBowyMississippi River and Tributaries Regional Technical Center
Vicksburg, Mississippi USA
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Properties of Water
Molecular polarity
molecules bond together
form open tetrahedral network for ice
Cohesionattraction to other water molecules
Adhesion
attraction to surfaces
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Biogeochemical Elements
See Hopkin's Cafe? Mighty good!
for plant nutrients:
C, H, O, P, K, N, S, Ca, Fe, Mg
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Redox Reactions Reduction-oxidation reactions
Electron-transfer reactions
4 Fe + 3 O22 Fe2O3 (oxidation)
2 Fe2O3+ 3 C3 CO2+ 4 Fe (reduction)
Gain or loss of electrons results in a change inan atoms oxidation state
Fe0Fe+3+ 3e-
O0+ 2e-O-2
Many elements (e.g., Fe, Mn, N, P, S) can havemultiple oxidation states
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Redox Reactions 2
All metal atoms are characterized by their
tendency to be oxidized, losing one or moreelectrons, forming a positively charged ion,called a cation.
during this oxidation reaction , the oxidation stateof the metal always increases from zero to apositive number, such as "+1, +2, +3...."dependingon the number of electrons lost
Fe0
Fe+3
+ 3e-
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Redox Reactions 3
The electrons lost by the metal are not
destroyed but gained by the nonmetal, which issaid to be reduced
As the nonmetal gains the electrons lost by the
metal, it forms a negatively charged ion, calledan anion
during this reduction reaction, the oxidation state ofthe nonmetal always decreases from zero to a
negative value (-1, -2, -3 ...) depending on thenumber of electrons gained
O0+ 2e-O-2
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Redox Reactions 4
Important to remember in oxidation-reduction
reactions is that the process of oxidation cannotoccur without a corresponding reductionreaction
oxidation must always be "coupled" with reductionelectrons that are "lost" by one substance must
always be "gained" by another as matter (such aselectrons) cannot be destroyed or created
hence, the terms lost or gained simply meanthat electrons are transferred from one particle toanother
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Electronegativity Over the years the definition of oxidation-
reduction has been broadened to includeprocesses which involve combinations of atomsin which there is no clearcut transfer ofelectrons between them.
An understanding of this behavior is providedby the concept of electronegativity.
According to this concept, each kind of atomhas a certain attraction for the electronsinvolved in a chemical bond.
Electron-attracting" power of each atom canbe listed numerically on electronegativity scale.
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Electronegativity 2
Electronegativity Values of Selected Elements
Metallic Elements Non-metallic Elements
Li Be C N O F
(1.0) (1.5) (2.5) (3.0) (3.5) (4.0)Na Mg Al P S Cl
(1.0) (1.2) (1.5) (2.1) (2.5) (3.0)
K Ca Sc Se Br(0.9) (1.0) (1.3) (2.4) (2.8)
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Electronegativity 3
Note the following trends:
metals generally have low electronegativity values,while nonmetals have relatively highelectronegativity values.
electronegativity values generally increase from leftto right within the Periodic Table of the elements.
electronegativity values generally decrease fromtop to bottom within each family of elements
within the Periodic Table.
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Electronegativity 4when atoms react with each other, they "compete"
for the electrons involved in a chemical bond.the atom with the higher electronegativity value,
will always "pull" the electrons away from the atomthat has the lower electronegativity value.
the degree of "movement or shift" of theseelectrons toward the more electronegative atom isdependent on the difference in electronegativities
between the atoms involved.
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Electronegativity 5
The situation is a bit more complex when only
nonmetal atoms are involvedas all nonmetals have similarly high electronegativity
values, it is unreasonable to assume that there will be atransfer of electrons between them in an oxidation-reduction
reactionconsequently valence electrons involved can no longer be
thought of as being "lost or gained" between atoms, butinstead, are only partially transferred, moving closer to thatatom which has higher electronegativity (and away from
atom of lower electronegativity)
this "shift" of electrons results in an unequal distribution ofcharge, as more electronegative atom becomes more"negative" and the atom of lower electronegativity becomes
more "positive"
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Electronegativity 6
The accurate determination of the distribution
of charge resulting from these "electron shifts"is very difficult, but guidelines have beendevised to simplify the process
in general, these guidelines assign the moreelectronegative atom a negative oxidation state, andthe atom with the lower electronegativity, a positiveoxidation state
these guidelines are at best, arbitraryapproximations, and in some instances may have tobe supplemented by additional methods
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Shallow Redox Reactions
Redox rxns different in wetlands or fringes
than in deeper parts of lakes/pondsFringes and wetlands are shallow
no thermoclineno dense cold deoxygenated water
some wetlands/fringes dry outaerobic processes
plants (or algae) can grow on bottom in shallowsaerobic root zone
Anaerobic/aerobic soil interface
leads toreductive/oxidative processes
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Shallow Redox Reactions 2
Anaerobic environment (saturated soil):
reduction processese-are donated
O atoms stripped off
H atoms added
Aerobic environment (root zone): oxidativeprocesses
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Shallow Redox Reactions 3 OxidationReduction
Fe+3Fe+2 (metal reactions result in
Mn+4Mn+2 Chemical Oxygen Demand)
Organic PSOPPO4-3HPO4
-2H2PO4-
insoluble inorganic P (complexes with Ca, Fe, Al;adsorption on clay/organic particles)
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Shallow Redox Reactions 4 OxidationReduction
Organic NNH3NH4+ (ammonification)
NO3-NO2
-NH4+ (nitrification)
NO3-
N2ON2 (denitrification)
CO3-2HCO3
-CO2C(H2O) (carbonate system)
C(H2O)CH4(methanogenesis)
SO4-2S-2(H2S)
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Global Change and Mercury
Global change and mercury.Krabbenhoft and Sunderland Science 2013;341:1457-1458.
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Acid Sulfate Soils
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Effects on Water Color
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Hydric Soils
Defined (USDA-NRCS) as soils that formed
under conditions of saturation, flooding, orponding long enough during the growingseason to develop anaerobic conditions in theupper part (Federal Register, July 13, 1994)
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Chemical Transformations
Anaerobic (reducing) conditions due to
presence of water Wetland soils both medium for storage and
chemical transformation
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Technical Cr iter iaHydric Soils
Mineral soils in several classifications:
Somewhat poorly drained and water table
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Hydric Soils Classif ication 2
All organic soils (Histosols) except Folists
Predominantly organic (mosses, herbaceousmaterial, wood or leaf litter)
bogs, moors, peats, or mucks
muck: >2/3 decomposed (Saprists)
peat:
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Hydric Soils Classif ication 3
Few soils consist of shallow organic material
resting on rock or rubblesaturated with water and contain >12-18% organic
carbon by (dry) weight
saturation only a few days: >20% organic carbon Histosols >50% organic matter by volume
usually saturated or nearly saturated for most of
year
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Organic/mineral Soil Comparison
Organic Mineral
Bulk Densities low high
(.2-.3g/cm3) (1-2g/cm3)
Hydraulic Conductivity low to high high
Cation Exchange Capacity dominated by H+ dominant major
cation: Ca++,
Mg++, K+, Na++
Nutrient Availability low high
Organic Content >20-35%
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Hydric Soil I dentif ication
Soil Color
indirect measure of other soil characteristics
Fe+3Fe+2; Mn+4Mn+2
redunhydrated iron oxide
yellowiron oxides
browniron oxides and organic matter
greypermanently saturated; reduced iron
easy to measure
Munsell color chart
in temperate climates, dark soils are relatively higher inorganic content
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Soil Color Elements of soil color descriptions are:
color name
Munsell notation
water state
physical state
example: "brown (10YR 5/3), dry, crushed andsmoothed"
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Soil ColorMunsell Notation Munsell notationis obtained by comparison
with a Munsell system color chartmost commonly used chart includes only about
one fifth of the entire range of hues
consists of about 250 different colored papers,or chips, systematically arranged on hue cardsaccording to their Munsell notations
system uses three elements of colorhue,value
, andchroma
to make up a colornotation
notation is recorded in the form: hue,value/chromafor example, 5Y 6/3
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Soil ColorWater State Color value of most soil material becomes
lower after moistening
Consequently, the water state of a sample isalways given
water state is either "moist" or "dry"
dry state for color determinations is air-dry andshould be made at the point where the colordoes not change with additional drying
color in moist state is determined onmoderately moist or very moist soil materialand should be made at the point where the colordoes not change with additional moistening
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H d i S il I d tif i ti 2
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Hydric Soil I dentif ication 2
Mottling
refers to repetitive color changes that cannot beassociated with compositional properties of the soil
mottles are described by quantity, size, contrast,
color, and other attributes in that orderredoximorphic features are a type of mottling that
is associated with wetness (alternating aerated andsaturated conditions)
these gray/brown/red spots are caused principallyby migration, depletion or concentration of Fe andMn within the soil
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H d i S il I d tif i ti 3
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Hydric Soil I dentif ication 3 Gleying
condition that develops when soil is wet for most ofthe year
soil matrix color is gray or bluish gray due totransformation of iron caused by prolonged
reducing conditionscharacteristic of very poorly drained soils
Specific soil ID often difficult
palustrine wetlands may be young withflooding/deposition/erosion
floodplain soils can have poorly developedcharacteristics
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