Section 2: Soil Organic Matter Flashcards
Forms of SOM
DOC/DOM(dissolved); POM (colloids&aggregates); MAOM (mineral associated organic matter)
Important functions of SOM
structure, climate, water, nutrients, toxins
Main elements in SOM
C, H, O, N, P, S
in what directions does carbon increase in the US
going north and going east
why do plowed soil have different OC%
because plowing exposes carbon and decreased the amount systematically
Living organisms (biota) impact SOM how potentially?
through exudates and bioturbation (earthworms mix the surface into the subsurface)
why does the rhizosphere play an important role in soil formation?
provides a large interface between soils, plants and organisms
What are the components of SOM
living organisms <5%; fresh residue <10%; decomposing OM 33-50%; Stabilized OM 33-50% (decomposing and stabilized depends on the environment)
What Is the typical composition of representative green-plant matter in SOM
water 75%; dry matter 25% (type of compounds – cellulose 45%, fats&waxes 2%, hemicellulose 18%, polyphenols 2%, lignin 20%, protein 8%, sugars and starches 5%; Elemental composition– carbon 42%, oxygen 42%, ash 8%, hydrogen 8%)
what is the most prevalent functional group in soil?
oxygen
soil forming processes: transformations
drive soil profile development
soil constituents are (bio)chemically and/or physically modified
What are the classifications of SOM?
Microbial biomass, plant roots, unaltered debris an transformed products (recognizable compounds/biomolecules) and amorphous polymers/humic substances)
biomolecules in soils: organic acids
LMW monomers
“small molecules” “metabolites”
microorganisms and plant root exudates
concentration range in the soil solution is 0.01-5 micro mol/L
biomolecules in soils: amino acids (to proteins)
LMW monomers
concentration range in soil solution is 0.05-0.6 micro mol/L
neutral (glycine, alanine), acidic (aspartic acid, glutamic acid), basic (arginine, lysine)
amino acids combine to form?
peptide and proteins (condensation polymers)
siderophores
“specialized metabolites”
exuded by bacteria/fungi
small multi dentate organic molecules that bind ferric iron (Fe3+) with high affinity
produced under low iron conditions; in addition, outer membrane receptor proteins are produced that facilitate iron transfer into the bacterium
transient: when sufficient levels have been acquired, biosynthesis stops
what kind of siderophore is used by plants?
phytosiderophore because it is generally smaller than the bacteria ones and typically contain N
specialized metabolites (secondary metabolites)
phenols/flavins/many others
non-volatile compounds in root exudate or emitted as volatile organic compounds (VOCs)
specialized metabolites (secondary metabolites): chemistry & purpose
release/exudation of specialized plant metabolites (LMWO) that change the soil chemical environment and/or directly interact and dissolve mineral phases that contain nutrients
chemistry: act via acid-dissolution, chelation and/or reduction
purpose: increase availability of nutrients for plant uptake, defense, chemical communication
carbohydrates
most plentiful of plant organic compounds and monosaccharides polymerize to form oligosaccharides and polysaccharides
cellulose
condensation polymer of glucose (&other sugars) , most abundant plant (cell wall) residues
Cellulose has hydrogen bonds, when is it more difficult to breakdown?
when there is more hydrogen bonds, it is harder to break down because it has intramolecular and intermolecular bonding
cellulase
enzymatic decomposition (bacteria and fungi)
lignin
a very stable component of plant cell walls
what compound forms lignin?
polycondensation of phenolic and related alcoholic compounds
why is lignin resistant to decay?
because it does not have hydrolysable bonds
is cellulose or lignin’s decay rate generally faster?
cellulose generally have a faster rate of decay.
does lignin have H-bonds?
not very much but it has a lot of chemical bonds
lipids
operationally defined by sparing solubility in water and their solubility in organic solvents
hydrophobic or amphiphilic and contain ester functional groups
some are structural components of cell membranes; store energy
include fatty acids, phospholipids, fats, waxes
1-6% SOM, 4.2-6.2% SOC
how long do lipids live in aerobic soil?
they are generally short-lived but some fats and waxes are found to accumulate in highly acidic or anaerobic soils
proteins
has intramolecular H-bonds, intermolecular bonds and has nitrogen
self-aggregation of organic molecules protects parts of the molecule
DNA
source of phosphorus, 100-1000 times larger than proteins
sugar phosphate backbone
polar, hydrophilic phospates
deoxyribose sugar
helix structure is very strong
chlorophyll
on chlorophyll a, if you cut the tail, you get chlorophyllide a which can be oxidized to oxidized tetrapyrrole where hydrogens are flexible and can grad iron or other cations
does bacteria, algae or fungi have more N-containing compounds in their dry matter?
bacteria! It has 50-60% N-containing compounds
why do tree leaves and herbaceous plants have high C:N ratios?
because of the cell walls, cellulose, lignin have lots of C which makes the ratio higher
what is the decomposition rate of plant residues from rapid to slow?
sugars, starches, simple proteins – crude proteins – hemicellulose – cellulose – fats and waxes – lignin’s and phenolic compounds
how to extract humid substances
Soil + NaOH (pH 11) – goes into humic unextracted and humic & fulvic acids to solution – acidification – fulvic acid remains in solution and humid acid precipitates
humin to humic acid to fulvic acid has what trend in molecular weight, carbon content, nitrogen content, and resemblance to lignin
decreasing
humin to humic acid to fulvic acid has what trend in oxygen content and acid content (CEC)?
increasing
van Krevelen Diagram
H/C ratio (degree of aromaticity) on the y-axis and O/C ratio (degree of oxidation) on the x-axis
van Krevelen Diagram: black carbon
high aromaticity and low hydrophilicity
van Krevelen Diagram: fulvic acid
mid aromaticity and high hydrophilicity
van Krevelen Diagram: as H/C increases what happens to aromaticity?
it decreases
van Krevelen Diagram: as O/C increases what happens to hydrophilicity?
it increases
C/N ratio of SOM is indicative of what? what is the value for fresh residue? humic substances?
the degree of decomposition
smaller C/N ratio is more decomposed
fresh: 25:100
humic: 10-20
van Krevelen Diagram: lipid-like
high H/C ratio, low O/C ratio
van Krevelen Diagram: protein/peptide
high H/C, mid O/C
van Krevelen Diagram: aminosugar-like
mid-high H/C and O/C
van Krevelen Diagram: cellulose-like
high H/C and O/C
van Krevelen Diagram: unsaturated hydrocarbon-like
mid H/C and low O/C
van Krevelen Diagram: lignin-like
mid H/C and O/C
van Krevelen Diagram: tannin-like
mid H/C and high O/C
humification (controversial)
the abiotic polymerization into new covalently coded carbon compounds in soils
the modification of lignin includes
loss of methyl groups, generation of hydroxyphenols and oxidation of aliphatic chains
what are the principal humus-forming compound?
polyphenols (quinones, amino compounds – polymerize readily)
why is the theory of humus formation important?
could provide clues as to their structures, greater comprehension of the carbon cycle, changes that occur when plant residues & organic waste decompose
what is SOM: polyfunctionality
variety of functional groups and a broad range of functional group reactivity, representative of heterogeneous mixture of interacting compounds
what is SOM: macromolecular charge
macromolecular framework with anionic character with the resultant effects on functional group reactivity and molecular conformation
what is SOM: hydrophilicity and hydrophobicity (polar and non-polar chemical components)
functional groups forming H-bonds with water and hydrophobic interactions
what is SOM: structural lability
the capacity to associate intermolecularly and to change molecular conformation in response to changes in pH, redox, electrolyte concentration and functional group binding
why does OM persist in soils?
inherent chemical recalcitrance, persistence varies with climate and soil properties, chemical stabilization via bonding (supramolecular associations, complexation, DNA-protein interactions)
what size is a macro and micro aggregates
macro - 250-2000 micrometers
micro - 53-250 micrometers
what is missing from SOM degradation susceptibility?
redox chemistry in anaerobic situations, methanogens, using N as an electron accepter when O is restricted
structure of century ecosystem model
based on turnover rates of SOM pools, evaluate the effects on environmental change, evaluate changes due to management practices
what are the 3 SOM “pools”?
Active, slow and passive
SOM pool: active
live microbes and their by-products (0.5 to 5 year turnover
SOM pool: slow
physically and chemically protected (10-50 year turnover
SOM pool: passive
physically protected or chemically resistant (800-1200 year turnover)
