Soil Organic Matter
Biomolecules
Organic Acids
Carbohydrates
Other
Humic Substances
Composition
Formation
Cation Exchange
Reaction with Organics
Reaction with Minerals
dC / dt = -kC
dC / dt = -kC + A
Active OM (t½ ~ 3 yr)
microbial biomass and
short-lived organics
Slow OM (t½ ~ 30 yr)
physically / chemically
protected / resistant
Passive OM (t½ ~ 300+ yr)
Biomolecules
Organic Acids
Aliphatic
Source of acidity for
mineral weathering
Facilitated by complex
formation, M – A
[HA] in soil solution ranges,
0.00001 – 0.005 M
Would you expect long or
short 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 form
peptides (polymers)
~ ½ soil N in amino acids,
especially as peptides
Carbohydrates
Monosaccharides
May contain acidic or basic
substituents
Polysaccharides
Monosaccharides are polyalcohols
Phenols are aromatic alcohols
Coniferyl alcohol is constituent of
Lignin
Along with cellulose, a possible
precursor of humic substances
Other Biomolecules
P-containing species
Inositol phosphates
Nucleic acids
S-containing species
Amino acids
Phenols
Polysaccharides
Lipids
Catch-all term for group characterized by
solubility in organic solvents
Soil lipids primarily fats, waxes and resins
Fats are esters of glycerol
Waxes similar but not derived from glycerol
Other 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 other
being biomolecules
HS unique to soil, structurally different
from biomolecules and highly resistant to
decomposition
Composition
HS include fulvic acids, humic acids and humin
 Calculate an average composition for humic acid of C187H186O89N9S
and 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 acids
to 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, and
lability 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)
(2)
(3)
(4)
Decomposition of biomolecules into simpler structures
Microbial metabolism of the simpler structures
Cycling of C, H, N, and O between soil organic matter and microbial biomass
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 of
dissociation 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 is
seen to be the sum of two
Langmuir equations,
S = kSMax [A] / (1 + k[A])
where S is adsorbed concentration, SMax is maximum
adsorbed concentration per unit
mass and k is an adsorption
affinity coefficient.
This adsorption model is widely
applicable 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
Protonation
δ+
δ-NH2 –H—O-
Anion exchange
-COO- and Φ-O- are exchangeable species
Bridging
-COO- coordinated with H2O which is also
coordinated 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 clay
particles into stable aggregates. Speculate why.
5. Humic substances do not associate with 2:1 clay minerals in the interlayer
region unless pH < 3. Give two reasons why.
10. Tetrachloroethylene solvent may contaminate groundwater if leached. Given
a water solubility of 5 mol m-3 (0.005 M), estimate KD and discuss whether it
is 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 = θC / t + ρ S / t
S = KDC
M / t = θC / t + ρKD C / t
θC / 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 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
lative Total Concentration
RF = 1 + (1.44 / 0.46) x 0.95 = 4
1.0
KD
0.8
KD
KD
0.6
0.4
0.2
KD