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

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KD = 0.000, R = 1

KD = 0.333, R = 2

KD = 1.000, R = 4

KD = 13.000, R = 40