Soil Organic Matter Biomolecules Organic Acids Carbohydrates Other Humic Substances Composition Formation Cation Exchange Reaction with Organics Reaction with Minerals
Jan 24, 2016
Soil Organic Matter
Biomolecules
Organic AcidsCarbohydratesOther
Humic Substances
CompositionFormation
Cation Exchange
Reaction with Organics
Reaction with Minerals
dC / dt = -kC
dC / dt = -kC + A
Active OM (t½ ~ 3 yr)
microbial biomass andshort-lived organics
Slow OM (t½ ~ 30 yr)
physically / chemicallyprotected / resistant
Passive OM (t½ ~ 300+ yr)
Biomolecules
Organic Acids
Aliphatic
Source of acidity formineral weathering
Facilitated by complexformation, M – A
[HA] in soil solution ranges,0.00001 – 0.005 M
Would you expect long orshort half lives?
Aromatic Acids
[HA] ranges 0.00005 – 0.00050 M
Amino Acids
[HA] ranges 0.00005 – 0.00060 M
Neutral, acidic and basic forms
React by condensation to formpeptides (polymers)
~ ½ soil N in amino acids, especially as peptides
Carbohydrates
Monosaccharides
May contain acidic or basicsubstituents
Polysaccharides
Monosaccharides are polyalcohols
Phenols are aromatic alcoholsConiferyl alcohol is constituent of
Lignin
Along with cellulose, a possible precursor of humic substances
Other Biomolecules
P-containing species
Inositol phosphatesNucleic acids
S-containing species
Amino acidsPhenolsPolysaccharides
Lipids
Catch-all term for group characterized bysolubility in organic solvents
Soil lipids primarily fats, waxes and resinsFats are esters of glycerolWaxes similar but not derived from glycerolOther soil lipids include steroids and terpenes
Humic Substances
Definitions
Soil organic matter includes living biomass,residue and humus (dark and colloidal)
Humic substances (HS) are major component of humus, the otherbeing biomolecules
HS unique to soil, structurally different from biomolecules and highly resistant todecomposition
Composition
HS include fulvic acids, humic acids and humin
Calculate an average composition for humic acid of C187H186O89N9Sand for fulvic acid, C135H182O95N5S2
Ranges of MWs, 2,000 to 50,000 for fulvic acids, and + 50,000 for humic acids
High content of dissociable H (carboxylic and phenolic groups)
Assuming full dissociation, compare the CECs of average humic and fulvic acidsto that of smectite.
See Table 3.4 (text).
Sums of masses C + H + N + S + O for HA and FA are both ~ 1 kg.
Therefore, charges per mass are ~6.7 and 11.2 mole / kg.
In contrast (Table 2.3), the charge per mass of smectite ~ 0.85 mole / 0.725 kg,or about 1/5 to 1/10 of that for HA and FA.
Carboxyl > phenol > alcohol > quinone and keto (carbonyl) > amino > sufhydryl (SH)
Polyfunctionality of individual humic molecules leads to intricate structural complexities due to covalent cross-linkages, electrostatic and H-bonds, andlability depending on solution pH, ionic strength and Eh
Biochemistry of Humic Substance Formation
Formation of HS not understood but generally thought to involve 4 stages
(1) Decomposition of biomolecules into simpler structures(2) Microbial metabolism of the simpler structures(3) Cycling of C, H, N, and O between soil organic matter and microbial biomass(4) Microbially mediated polymerization of the cycled materials
Lignin (lignin-protein) theory
(Waxman, 1932)
Lignin is incompletely used by microbes and residual part makes up HS
Polyphenol theory
These from either from lignin decomposition or derived by microbes from other sources
Oxidation of polyphenols to quinones leads to ready addition of amino compounds and development of structurally large condensation products
Sugar-amine condensation theory
Simple reactants derived from microbial decomposition undergo polymerization
All may occur but relative importance is site-specific
Cation Exchange
Can be determined by measuring H+ released by reaction with Ba2+
2SH(s) + Ba2+(aq) = S2Ba(s) + 2H+(aq)
Fast kinetics of exchange, limited only by diffusion
CEC of humic substances is pH dependent and the extent ofdissociation as a function of pH can be determined by titration
Titration curve, also called formation function for proton binding, can be modeled by expressions like
nH = (b1K110-pH) / (1 + K110-pH) + (b2K210-pH) / (1 + K210-pH)
δnH = [(nH – [H+]V) – (nOH – [OH-]V) ] / m
δnH0 = – (nOH – [OH-]0V0) / m
δnH1 = [(nH1 – [H+]V1) – (nOH – [OH-]1V1) ] / m
nH1 = δnH1 – δnH0
= [(nH1 – [H+]V1) – ([OH-]0V0 – [OH-]1V1)] / m
Cumulative H+ adsorption as function of [H+] or pH.
nH = (b1K110-pH) / (1 + K110-pH) + (b2K210-pH) / (1 + K210-pH)
with 10-pH = [H+], what have we?
Making the substitution, nH isseen to be the sum of twoLangmuir equations,
S = kSMax [A] / (1 + k[A])
where S is adsorbed concen-tration, SMax is maximumadsorbed concentration per unitmass and k is an adsorption affinity coefficient.
This adsorption model is widelyapplicable in soils.
In turn, pH buffering by soil organic matter can be expressed in terms of nH.
The acid-neutralizing capacity is ANC = (nHtotal - nH) CHumus + [OH-] – [H+]
dANC / dpH = buffer intensity
Where steepest, greatest pH buffering
ANC = (nHtotal - nH) CHumus + 10pH-14 – 10-pH
where nH = (b1K110-pH) / (1 + K110-pH) + (b2K210-pH) / (1 + K210-pH)
So buffer intensity, dANC / dpH is awkward to calculate.
Reaction with Organics
Positively and negatively affect mobility of organics in soil
Adsorption by solid phase humic substances retards mobility whereas complex formation with soluble fulvic acids facilitates mobility
Term “facilitated transport” was fairly recently used and an active research area
Examples of retention
Cation exchange
SH + NR4+ = SNR4 + H+
H-bonding involving C=O, -NH2, -OH and even -COOH
Dipole – dipole interaction
van der Waals, induced dipoles
Lead to high affinity of nonpolar species for soil organic matter
Affinity described by a distribution coefficient
Kd = S / C
where S is adsorbed concentration and C is solution concentration
Commonly, the distribution coefficient is normalized with respect to soil organic matter to give
KOM = Kd / fOM
Hydrophobic interactions of nonpolar solutes and soil organic matter are inversely related to the water solubility of the nonpolar solute.
Approximately,
log KOM = a – b log Sw
where Sw is water solubility
Reaction with Minerals
Cation exchange -NH3+ is an exchangeable species
δ+ δ-Protonation -NH2 –H—O-
Anion exchange -COO- and Φ-O- are exchangeable species
Bridging -COO- coordinated with H2O which is alsocoordinated with cation adsorbed on mineral
-COO- M+ with M+ adsorbed on mineral
Ligand exchange -COO- + +H2O-Al = -COO-Al- + H2O
Hydrogen bonding O—H --- O-Si
Dipole-dipole
van der Waals attraction between induced dipoles
Let’s answer a couple of questions and do a problem.
4.Polysaccharides are more effective than humic substances in binding clayparticles into stable aggregates. Speculate why.
5.Humic substances do not associate with 2:1 clay minerals in the interlayerregion unless pH < 3. Give two reasons why.
10. Tetrachloroethylene solvent may contaminate groundwater if leached. Givena water solubility of 5 mol m-3 (0.005 M), estimate KD and discuss whether itis relatively mobile or immobile in soil. Assume 20 g humus per kg soil.
log (KOM) = 2.118 – 0.729 log (S)
KOM = 47.69 kgSOLN / kgOM = 47.69 L / kgOM
KD = KOM x fOM = 47.69 L / kgOM x 0.02 kgOM / kgSoil
KD = 0.95 L / kgSoil
Convective-Dispersive Model for Solute Transport
M / t = θD 2C / z2 – q C / z
M = θC + ρS
M / t = θC / t + ρ S / t
S = KDC
M / t = θC / t + ρKD C / t
θC / t + ρKD C / t = θD 2C / z2 – q C / z
(1 + ρKD / θ) C / t = D 2C / z2 – v C / z
Retardation Factor
RF = (1 + ρKD / θ)
If ρ = 1.44 kg dm-3 and soil saturated, θ = 0.46 so that
RF = 1 + (1.44 / 0.46) x 0.95 = 4
RF when there is no sorption is 1
Movement inversely related to RF,
distance at RF = X relative to distance at RF = 1 is 1 / X
0 20 40 60 80 100
Depth in Soil
0.0
0.2
0.4
0.6
0.8
1.0
Rela
tiv
e T
otal
Co
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KD = 0.000, R = 1
KD = 0.333, R = 2
KD = 1.000, R = 4
KD = 13.000, R = 40