Electrochemical Controls

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Electrochemical Controls on the Dynamics
of Macronutrient Anions and Low Molecular
Weight Organic Acids in Variable-Charge Soils
Research Proposal by Brian D.
Strahm University of Washington College of
Forest Resources
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Physical Properties
Soil Processes
Soil Function
Chemical Properties
Abiotic
Biotic
Biological Properties
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(Adapted from Brady, 2002)
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Soils adsorb cations, but not anions
Brilliant!
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Variable-Charge Soils
(NRCS, 2006)
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Variable-Charge Soils
(NRCS, 2006)
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U.S. Forest Cover
(USGS, 2006)
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Variable-Charge Soil Orders
Forest Cover
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variable-charge soil \ver-e- -b l chärj
soil\ n. soil in which electrostatic charge is
dependent upon pH

e
e
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(Sposito, 1989)
(Singh and Uehara, 1999)
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(Sposito, 1989)
(Singh and Uehara, 1999)
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Plants
Uptake
Release
Leaching
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Fall River LTSP Study
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(data from Flaming, 2002)
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(Brady and Weil, 2002)
H H
COO-
Dissolved Organic Matter
?
O-
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(Brady and Weil, 2002)
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• Improves structure and aggregation
• Decreases bulk density ? Increases porosity
• Increases water holding capacity

Physical Properties

• Increases cation exchange capacity (CEC)
• Buffers pH change
• Improves solubility of plant nutrients

Chemical Properties

• Slowly available C source to
• microbial community
• Storehouse of slowly available nutrient capital

Biological Properties
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Nonhumic Substances
Fulvic Acid
Humic Substances
Humic Acid
Humin
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Plants
CO2
Respiration
Uptake
Release
Leaching
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Plants
CO2
Respiration
Uptake
Release
pH?
Leaching
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• Nonhumic Substances
• compounds belonging to known biochemical classes
• ex. - organic acids, amino acids,
  carbohydrates, fats, waxes, and resins
• Low Molecular Weight Organic Acids (LMWOAs)
• lt 250-300 amu
• constantly exuded by roots (mmol L-1)
• N Compounds
• gt 100 N compounds contributing to SOM
• ALL lt 300 amu
• Amino Acids 30-45 of N in soils (largest
  fraction)

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Total AA Free AA
Soil 30-45 2
Solution 50-75 10-20
(Stevenson, 1994)
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Amine
Carboxylic Acid
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Amino Acid
COO-
NH2
pH gt 3
- H
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Amino Acid
COO-
NH2
- H
pH gt 3
pH lt 9
H
Amino Acid
COOH
NH3
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3 lt pH lt 9 ?
Amino Acid
COO-
NH3
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The position at which an ion adsorbs at a
solid/aqueous electrolyte interface, and the
magnitude of the forces which cause that
adsorption, are characteristic of the ion and
the interface.
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• b and Nmax as dependent variables
• Soil physical and chemical factors as
  independent variables
• particle size
• mineralogy
• pH
• charge characteristics

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H1 The soils of the Fall River LTSP study
exhibit variable-charge characteristics and
have the capacity to sorb NO3- at ambient pH
values.
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H1 NO3- Sorption at Fall River
Nmax
b
AEC
Net Charge
CEC
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• Additional Analyses
• Selective Dissolution Analysis (Fe, Al, Si)
• Pyrophosphate organic
• Acid Oxalate organic noncrystalline
• Dithionite Citrate organic noncrystalline
  crystalline
• Particle Size Analysis
• Soil pH
• Soil C
• Methodological Considerations
• Microbial Transformations ? Inhibit with toluene

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H1 The soils of the Fall River LTSP study
exhibit variable-charge characteristics and
have the capacity to sorb NO3- at ambient pH
values. H2 The sorption of macronutrient anions
(NO3-, SO4-2 and PO4) at a fixed pH is a
function of soil mineralogy.
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H2 Macronutrient Anion Sorption
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H2 Macronutrient Anion Sorption
Nitrate (NO3-)
Sulfate (SO4-2)
Phosphate (H2PO4-)

• Methodological Considerations
• Ion Competition ? Presaturate soils with KCl
• Varied pH regimes ? adjust to 4.0

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H1 The soils of the Fall River LTSP study
exhibit variable-charge characteristics and
have the capacity to sorb NO3- at ambient pH
values. H2 The sorption of macronutrient anions
(NO3-, SO4-2 and PO4) at a fixed pH is a
function of soil mineralogy. H3 The sorption
of LMWOAs is a function of soil mineralogy, pH
and the charge characteristics of the acids.
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• 3 x 3 x 3 Factorial Design
• Soils
• pH 4
• 6
• 8
• LMWOAs
• Amino Acid
• Mono-carboxylic
• Di-carboxylic

WA - Grove
WA - Boistfort
NC - Cecil
Puerto Rico
Hawaii
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Glycine (MW 75.07)
Propionic Acid (MW 74.08)
Malonic Acid (MW 104.06)
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1
-1
0 (1 and -1)
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-1
0
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-1
-2
0
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SOM ?13C -27

• Methodological Considerations
• Microbial Transformations ? Treat soils with
  HgCl2
• Ion Competition ? Presaturate soils with KCl
• Define specific pH regimes
• C analysis ? 13C as a proxy

Final Soln
Added C ?13C 15
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H1 The soils of the Fall River LTSP study
exhibit variable-charge characteristics and
have the capacity to sorb NO3- at ambient pH
values. H2 The sorption of macronutrient anions
(NO3-, SO4-2 and PO4) at a fixed pH is a
function of soil mineralogy. H3 The sorption
of LMWOAs is a function of soil mineralogy, pH
and the charge characteristics of the acids.
H4 The desorption and mineralization of LMWOAs
is a function of their affinity to form a
complex with the solid soil matrix.
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Factor Level LMWOA Glycine NH3
- CH2 - (COOH) Propionic Acid CH3 - CH2 -
(COOH) Malonic Acid (HOOC) - CH2 -
(COOH) Soils WA- Grove (Matlock) Entisol WA -
Boistfort (Fall River) Andisol NC -
Cecil Ultisol pH
SORPTION Generate isotherms for the
3 x 3 x 3 factorial combination
MA -
Soil /-
PA 0
Gly /-
4
MA -/--
Soil -
PA 0/-
Gly /-
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MA --
Soil --
PA -
Gly /-
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DRY
WET
DRY
WET
DESORPTION Generate isotherms and calculate k
for analyte saturated soil
MINERALIZATION Generate decomposition curves and
calculate k based on CO2 efflux
Successive washes for Water Extractable LMWOAs
Laboratory Incubation for Mineralization Rates
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First-order rate reaction At Aoe-kt
Mineralization 48h incubation Desorption Water
extractable
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Physical Properties
Soil Processes
Soil Function
Chemical Properties
Abiotic
Biotic
Biological Properties