nuap 2014 3. biogeochemistry

8/12/2019 NUAP 2014 3. Biogeochemistry

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Ecohydrology of Lake Titicaca 2014

Biogeochemistry

Dr. Paul J. DuBowyMississippi River and Tributaries Regional Technical Center

Vicksburg, Mississippi USA
http://www.cies.org/Fulbright/


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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



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.



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)



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.



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.



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"



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



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



Shallow Redox Reactions 2

Anaerobic environment (saturated soil):

reduction processese-are donated

O atoms stripped off

H atoms added

Aerobic environment (root zone): oxidativeprocesses



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)



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)



Global Change and Mercury

Global change and mercury.Krabbenhoft and Sunderland Science 2013;341:1457-1458.



Acid Sulfate Soils



Effects on Water Color



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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nuap 2014 3. biogeochemistry

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