Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Soil Organic MatterBiomoleculesOrganic AcidsCarbohydratesOtherHumic SubstancesCompositionFormationCation ExchangeReaction with OrganicsReaction with MineralsdC / dt = -kCdC / dt = -kC + AActive OM (t½ ~ 3 yr)microbial biomass andshort-lived organicsSlow OM (t½ ~ 30 yr)physically / chemicallyprotected / resistant Passive OM (t½ ~ 300+ yr)BiomoleculesOrganic AcidsAliphaticSource of acidity formineral weatheringFacilitated by complexformation, M – A [HA] in soil solution ranges,0.00001 – 0.005 MWould you expect long orshort half lives?Aromatic Acids[HA] ranges 0.00005 – 0.00050 MAmino Acids[HA] ranges 0.00005 – 0.00060 MNeutral, acidic and basic formsReact by condensation to formpeptides (polymers)~ ½ soil N in amino acids, especially as peptidesCarbohydratesMonosaccharidesMay contain acidic or basicsubstituentsPolysaccharidesMonosaccharides are polyalcoholsPhenols are aromatic alcoholsConiferyl alcohol is constituent of LigninAlong with cellulose, a possible precursor of humic substancesOther BiomoleculesP-containing speciesInositol phosphatesNucleic acidsS-containing speciesAmino acidsPhenolsPolysaccharidesLipidsCatch-all term for group characterized bysolubility in organic solventsSoil lipids primarily fats, waxes and resinsFats are esters of glycerolWaxes similar but not derived from glycerolOther soil lipids include steroids and terpenesHumic SubstancesDefinitionsSoil organic matter includes living biomass,residue and humus (dark and colloidal)Humic substances (HS) are major component of humus, the otherbeing biomoleculesHS unique to soil, structurally different from biomolecules and highly resistant todecompositionCompositionHS include fulvic acids, humic acids and humin Calculate an average composition for humic acid of C187H186O89N9Sand for fulvic acid, C135H182O95N5S2Ranges 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 EhBiochemistry of Humic Substance FormationFormation 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 materialsLignin (lignin-protein) theory (Waxman, 1932)Lignin is incompletely used by microbes and residual part makes up HSPolyphenol theoryThese 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 productsSugar-amine condensation theorySimple reactants derived from microbial decomposition undergo polymerizationAll may occur but relative importance is site-specificCation ExchangeCan 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 diffusionCEC 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 likenH = (b1K110-pH) / (1 + K110-pH) + (b2K210-pH) / (1 + K210-pH)δnH = [(nH – [H+]V) – (nOH – [OH-]V) ] / mδnH0 = – (nOH – [OH-]0V0) / mδnH1 = [(nH1 – [H+]V1) – (nOH – [OH-]1V1) ] / mnH1 = δnH1 – δnH0 = [(nH1 – [H+]V1) – ([OH-]0V0 – [OH-]1V1)] / mCumulative 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 intensityWhere steepest, greatest pH bufferingANC = (nHtotal - nH) CHumus + 10pH-14 – 10-pHwhere nH = (b1K110-pH) / (1 + K110-pH) + (b2K210-pH) / (1 + K210-pH) So buffer intensity, dANC / dpH is awkward to calculate.Reaction with OrganicsPositively and negatively affect mobility of organics in soilAdsorption by solid phase humic substances retards mobility whereas complex formation with soluble fulvic acids facilitates mobilityTerm “facilitated transport” was fairly recently used and an active research areaExamples of retentionCation exchangeSH + NR4+ = SNR4 + H+H-bonding involving C=O, -NH2, -OH and even -COOHDipole – dipole interactionvan der Waals, induced dipolesLead to high affinity of nonpolar species for soil organic matterAffinity described by a distribution coefficientKd = S / Cwhere S is adsorbed concentration and C is solution concentrationCommonly, the distribution coefficient is normalized with respect to soil organic matter to giveKOM = Kd / fOMHydrophobic 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 Swwhere Sw is water solubilityReaction with MineralsCation exchange -NH3+ is an exchangeable species δ+ δ-Protonation -NH2 –H—O- Anion exchange -COO- and Φ-O- are exchangeable speciesBridging -COO- coordinated with H2O which is alsocoordinated with cation adsorbed on mineral-COO- M+ with M+ adsorbed on mineralLigand exchange -COO- + +H2O-Al =