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Soil Organic Matter Biomolecules Organic Acids Carbohydrates Other Humic Substances Composition

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 (t ½ ~ 3 yr) microbial biomass and short-lived organics

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Soil Organic Matter Biomolecules Organic Acids Carbohydrates Other Humic Substances Composition

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  1. Soil Organic Matter Biomolecules Organic Acids Carbohydrates Other Humic Substances Composition Formation Cation Exchange Reaction with Organics Reaction with Minerals

  2. 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)

  3. 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?

  4. 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

  5. Carbohydrates Monosaccharides May contain acidic or basic substituents Polysaccharides

  6. Monosaccharides are polyalcohols Phenols are aromatic alcohols Coniferyl alcohol is constituent of Lignin Along with cellulose, a possible precursor of humic substances

  7. Other Biomolecules P-containing species Inositol phosphates Nucleic acids S-containing species Amino acids Phenols Polysaccharides

  8. 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

  9. 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

  10. 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.

  11. 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.

  12. 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

  13. 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

  14. Lignin (lignin-protein) theory (Waxman, 1932) Lignin is incompletely used by microbes and residual part makes up HS

  15. 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

  16. Sugar-amine condensation theory Simple reactants derived from microbial decomposition undergo polymerization All may occur but relative importance is site-specific

  17. 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

  18. 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)

  19. 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.

  20. 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 concen- tration, SMax is maximum adsorbed concentration per unit mass and k is an adsorption affinity coefficient. This adsorption model is widely applicable in soils.

  21. 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.

  22. 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

  23. 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

  24. 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 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

  25. Let’s answer a couple of questions and do a problem. • Polysaccharides are more effective than humic substances in binding clay • particles into stable aggregates. Speculate why. • 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.

  26. 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

  27. 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 = 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

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