SOIL ORGANIC MATTER Biomolecules Organic Acids; Carbohydrates; Other Humic Substances
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SOIL ORGANIC MATTER
Biomolecules
Organic Acids; Carbohydrates; Other
Humic Substances
Composition - Formation
Cation Exchange
Reaction with Organics
Reaction with MineralsSUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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)
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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?
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Karbohydrat
Monosaccharides
May contain acidic or basicsubstituents
Polysaccharides
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Monosaccharides are polyalcohols
Phenols are aromatic alcohols
Coniferyl alcohol is constituent of
Lignin
Along with cellulose, a possible
precursor of humic substances
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Other Biomolecules
P-containing species
Inositol phosphatesNucleic acids
S-containing species
Amino acidsPhenols
Polysaccharides
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Humic SubstancesDefinitions
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.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
CompositionHS 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.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Table 3.4 :
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.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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 .
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Lignin (lignin-protein) theory (Waxman, 1932)
Lignin is incompletely used by microbes and residual part makes up HS
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Sugar-amine condensation theory
Simple reactants derived from microbial decomposition undergo polymerization
All may occur but relative importance is site-specific
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
Cation 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 diffusion
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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)
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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 two
Langmuir equations,
S = kSMax [A] / (1 + k[A])
where S is adsorbed concen-tration, SMax is maximum
adsorbed concentration per unitmass and k is an adsorption
affinity coefficient.
This adsorption model is widelyapplicable in soils.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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 matterSUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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 dipolesSUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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.
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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 / tSUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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
tive
Tota
l Con
cent
ratio
n
KD = 0.000, R = 1KD = 0.333, R = 2KD = 1.000, R = 4KD = 13.000, R = 40
SUMBER: www.aem.lsu.edu/courses/agro2051/AGRO7055Index/Presentation2.ppt
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