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Soil Chemistry > Section 2: Soil Organic Matter > Flashcards

Section 2: Soil Organic Matter Flashcards

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1
Q

Forms of SOM

A

DOC/DOM(dissolved); POM (colloids&aggregates); MAOM (mineral associated organic matter)

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2
Q

Important functions of SOM

A

structure, climate, water, nutrients, toxins

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3
Q

Main elements in SOM

A

C, H, O, N, P, S

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4
Q

in what directions does carbon increase in the US

A

going north and going east

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5
Q

why do plowed soil have different OC%

A

because plowing exposes carbon and decreased the amount systematically

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6
Q

Living organisms (biota) impact SOM how potentially?

A

through exudates and bioturbation (earthworms mix the surface into the subsurface)

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7
Q

why does the rhizosphere play an important role in soil formation?

A

provides a large interface between soils, plants and organisms

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8
Q

What are the components of SOM

A

living organisms <5%; fresh residue <10%; decomposing OM 33-50%; Stabilized OM 33-50% (decomposing and stabilized depends on the environment)

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9
Q

What Is the typical composition of representative green-plant matter in SOM

A

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%)

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10
Q

what is the most prevalent functional group in soil?

A

oxygen

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11
Q

soil forming processes: transformations

A

drive soil profile development

soil constituents are (bio)chemically and/or physically modified

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12
Q

What are the classifications of SOM?

A

Microbial biomass, plant roots, unaltered debris an transformed products (recognizable compounds/biomolecules) and amorphous polymers/humic substances)

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13
Q

biomolecules in soils: organic acids

A

LMW monomers

“small molecules” “metabolites”

microorganisms and plant root exudates

concentration range in the soil solution is 0.01-5 micro mol/L

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14
Q

biomolecules in soils: amino acids (to proteins)

A

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)

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15
Q

amino acids combine to form?

A

peptide and proteins (condensation polymers)

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16
Q

siderophores

A

“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

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17
Q

what kind of siderophore is used by plants?

A

phytosiderophore because it is generally smaller than the bacteria ones and typically contain N

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18
Q

specialized metabolites (secondary metabolites)

A

phenols/flavins/many others

non-volatile compounds in root exudate or emitted as volatile organic compounds (VOCs)

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19
Q

specialized metabolites (secondary metabolites): chemistry & purpose

A

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

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20
Q

carbohydrates

A

most plentiful of plant organic compounds and monosaccharides polymerize to form oligosaccharides and polysaccharides

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21
Q

cellulose

A

condensation polymer of glucose (&other sugars) , most abundant plant (cell wall) residues

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22
Q

Cellulose has hydrogen bonds, when is it more difficult to breakdown?

A

when there is more hydrogen bonds, it is harder to break down because it has intramolecular and intermolecular bonding

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23
Q

cellulase

A

enzymatic decomposition (bacteria and fungi)

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24
Q

lignin

A

a very stable component of plant cell walls

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25
Q

what compound forms lignin?

A

polycondensation of phenolic and related alcoholic compounds

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26
Q

why is lignin resistant to decay?

A

because it does not have hydrolysable bonds

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27
Q

is cellulose or lignin’s decay rate generally faster?

A

cellulose generally have a faster rate of decay.

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28
Q

does lignin have H-bonds?

A

not very much but it has a lot of chemical bonds

29
Q

lipids

A

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

30
Q

how long do lipids live in aerobic soil?

A

they are generally short-lived but some fats and waxes are found to accumulate in highly acidic or anaerobic soils

31
Q

proteins

A

has intramolecular H-bonds, intermolecular bonds and has nitrogen

self-aggregation of organic molecules protects parts of the molecule

32
Q

DNA

A

source of phosphorus, 100-1000 times larger than proteins

sugar phosphate backbone

polar, hydrophilic phospates

deoxyribose sugar

helix structure is very strong

33
Q

chlorophyll

A

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

34
Q

does bacteria, algae or fungi have more N-containing compounds in their dry matter?

A

bacteria! It has 50-60% N-containing compounds

35
Q

why do tree leaves and herbaceous plants have high C:N ratios?

A

because of the cell walls, cellulose, lignin have lots of C which makes the ratio higher

36
Q

what is the decomposition rate of plant residues from rapid to slow?

A

sugars, starches, simple proteins – crude proteins – hemicellulose – cellulose – fats and waxes – lignin’s and phenolic compounds

37
Q

how to extract humid substances

A

Soil + NaOH (pH 11) – goes into humic unextracted and humic & fulvic acids to solution – acidification – fulvic acid remains in solution and humid acid precipitates

38
Q

humin to humic acid to fulvic acid has what trend in molecular weight, carbon content, nitrogen content, and resemblance to lignin

A

decreasing

39
Q

humin to humic acid to fulvic acid has what trend in oxygen content and acid content (CEC)?

A

increasing

40
Q

van Krevelen Diagram

A

H/C ratio (degree of aromaticity) on the y-axis and O/C ratio (degree of oxidation) on the x-axis

41
Q

van Krevelen Diagram: black carbon

A

high aromaticity and low hydrophilicity

42
Q

van Krevelen Diagram: fulvic acid

A

mid aromaticity and high hydrophilicity

43
Q

van Krevelen Diagram: as H/C increases what happens to aromaticity?

A

it decreases

44
Q

van Krevelen Diagram: as O/C increases what happens to hydrophilicity?

A

it increases

45
Q

C/N ratio of SOM is indicative of what? what is the value for fresh residue? humic substances?

A

the degree of decomposition

smaller C/N ratio is more decomposed

fresh: 25:100
humic: 10-20

46
Q

van Krevelen Diagram: lipid-like

A

high H/C ratio, low O/C ratio

47
Q

van Krevelen Diagram: protein/peptide

A

high H/C, mid O/C

48
Q

van Krevelen Diagram: aminosugar-like

A

mid-high H/C and O/C

49
Q

van Krevelen Diagram: cellulose-like

A

high H/C and O/C

50
Q

van Krevelen Diagram: unsaturated hydrocarbon-like

A

mid H/C and low O/C

51
Q

van Krevelen Diagram: lignin-like

A

mid H/C and O/C

52
Q

van Krevelen Diagram: tannin-like

A

mid H/C and high O/C

53
Q

humification (controversial)

A

the abiotic polymerization into new covalently coded carbon compounds in soils

54
Q

the modification of lignin includes

A

loss of methyl groups, generation of hydroxyphenols and oxidation of aliphatic chains

55
Q

what are the principal humus-forming compound?

A

polyphenols (quinones, amino compounds – polymerize readily)

56
Q

why is the theory of humus formation important?

A

could provide clues as to their structures, greater comprehension of the carbon cycle, changes that occur when plant residues & organic waste decompose

57
Q

what is SOM: polyfunctionality

A

variety of functional groups and a broad range of functional group reactivity, representative of heterogeneous mixture of interacting compounds

58
Q

what is SOM: macromolecular charge

A

macromolecular framework with anionic character with the resultant effects on functional group reactivity and molecular conformation

59
Q

what is SOM: hydrophilicity and hydrophobicity (polar and non-polar chemical components)

A

functional groups forming H-bonds with water and hydrophobic interactions

60
Q

what is SOM: structural lability

A

the capacity to associate intermolecularly and to change molecular conformation in response to changes in pH, redox, electrolyte concentration and functional group binding

61
Q

why does OM persist in soils?

A

inherent chemical recalcitrance, persistence varies with climate and soil properties, chemical stabilization via bonding (supramolecular associations, complexation, DNA-protein interactions)

62
Q

what size is a macro and micro aggregates

A

macro - 250-2000 micrometers

micro - 53-250 micrometers

63
Q

what is missing from SOM degradation susceptibility?

A

redox chemistry in anaerobic situations, methanogens, using N as an electron accepter when O is restricted

64
Q

structure of century ecosystem model

A

based on turnover rates of SOM pools, evaluate the effects on environmental change, evaluate changes due to management practices

65
Q

what are the 3 SOM “pools”?

A

Active, slow and passive

66
Q

SOM pool: active

A

live microbes and their by-products (0.5 to 5 year turnover

67
Q

SOM pool: slow

A

physically and chemically protected (10-50 year turnover

68
Q

SOM pool: passive

A

physically protected or chemically resistant (800-1200 year turnover)

Soil Chemistry (4 decks)

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