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GEOG 305 > Midterm Lectures 1-5 > Flashcards

Midterm Lectures 1-5 Flashcards

1
Q

What are the 3 phases of soil properties?

A

Solid phase: mineral and OM
Liquid phase: soil water and soil solution
Gaseous phase: soil gas amount and composition

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2
Q

What is the general composition of agricultural topsoil by volume?

A

25% air
25% water
12% OM
38% mineral

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3
Q

What is the general composition of organic soil by volume?

A

88% water
10% organic
1% air and mineral

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4
Q

What is the general composition of agricultural topsoil by dry mass?

A

95% mineral
5% organic

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5
Q

What is the general composition of organic soil by dry mass?

A

95% organic
5% mineral

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6
Q

What are the three important aspects of inorganic solid phase?

A

Particle size distribution
Soil structure
Mineralogy and chemistry

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7
Q

What are the four classes of particle size?

A

Gravel: >2 mm diameter
Sand: 2-0.02 mm diameter
Silt: 0.02-0.002 mm diameter
Clay: <0.002 mm (or 2 micrometer) diameter

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8
Q

What is Stoke’s Law, and what does it represent?

A

Stoke’s law is to determine the velocity of a falling particle. The larger the diameter of the particle, the larger the velocity.

V=h/t= (d^2(g)(Ds-Df))/18n

where,
V= velocity V of falling particle
Ds: density of particle
Df= density of fluid
d= particle diameter
g=gravitational force (9.81 N/kg)
n=viscocity of water at 20 degrees C

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9
Q

How do you measure soil texture in the lab?

A

Sample soil suspension after 46 sec. to get clay+silt fraction and after 8 hours to get clay fraction

  • Bring soil in suspension in solution that we know density
  • Soil particles will be disperesed –> all in suspension in solution
  • Measure the hieght
  • Sample the suspension, measure the mass of solid material retrieved
  • Depending on time, retrieve either clay and silt or clay
  • Larger diameter of particle –> faster velocity
  • Creates layers, with biggest diameter at the bottom (sand), and the finest particles at the top (clay)
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10
Q

How do you determine texture?

A

Using the triangular texture pyramid with silt, clay and sand fractions (%)

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11
Q

What is the importance of soil texture?

A

Influences soil infiltration rate, and thus generation of overland flow and soil erosion

Influences soil permeability and therefore drainage

Controls available water capacity of soil - ability to supply water to plants

Influences soil structure, allowing root growth and aeration

Provides cation exchange capacity for nutrient supply to plants and buffering against acid rain

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12
Q

What are the properties of sand, loam and clay for water holding capacity?

A

Sandy: low
Loamy: medium
Clay: high

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13
Q

What are the properties of sand, loam and clay for aeration and drainage?

A

Sand: well, rapid
Loam: moderate
Clay: poor, slow

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14
Q

What are the properties of sand, loam and clay for OM content?

A

Sand: low
Loam: medium
Clay: high

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15
Q

What are the properties of sand, loam and clay for decomposition rate?

A

Sand: fast
Loam: moderate
Clay: slow

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16
Q

What are the properties of sand, loam and clay for nutrient holding capacity?

A

Sand: small
Loam: moderate
Clay: high

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17
Q

What are the properties of sand, loam and clay for nutrient supplying power?

A

Sand: weak
Loam: moderate
Clay: high

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18
Q

What are the properties of sand, loam and clay for leaching of pollutants?

A

Sand: high
Loam: moderate
Clay: low

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19
Q

What are the properties of sand, loam and clay for sealing properties?

A

Sandy: poor
Loam: moderate
Clay: high

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20
Q

What are the properties of sand, loam and clay for shrinkage and swelling?

A

Sand: none
Loam: small
Clay: high

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21
Q

What are the properties of sand, loam and clay for compaction?

A

Sand: resists
Loam: moderately
Clay: easily

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22
Q

Place the surface area of coarse sand, montmorillonite clay, sild, fine sand, illite clay and kaoline clay in increasing order

A

Coarse sand: 0.01m^2g^-1
Fine sand: 0.1
Silt: 1
Kaoline clay: 5-100
Illite clay: 100-200
Montmorillonite clay: 700-1000

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23
Q

What is soil structure and what does it influence?

A

Describes the spatial arrangement of particles to complex aggregates, pores, and channels

Has a major influence on water and air movement as well as root growth

Influences the movement of soil macro- and meso- fauna

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24
Q

What are the major structural forms in soils?

A

Prismatic
Columnar
Angular blocky
Subangular blocky
Platy
Granular

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25
Q

What are the processes that lead to the development of soil structure?

A

Expansion/contraction upon wetting and drying

Root penetration

Earthworms and ingestion/excretion by other soil organisms
- Soil is usually copmacted with slimy substance from ingesting system of earthworms

Input and decomposition of SOM

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26
Q

What are the properties of maintaining soil structure?

A

High OM conent, especially input of fresh litter
- Promotes bonding between particles

Clay minerals - large, reactive surfaces with positive and negative charges

Presenec of Fe and Al oxides and hydroxides as cementing agents

Si coating on minerals - produces durpans in dry climates
- duripans are the really compacted later in subsoils where no water, roots, picks and axes can penetrates, it is stronger than many cements

CaCO3 deposits - produce caliche in semi-arid soils

Presence of flocculating cations (Ca2+ and Mg2+)

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27
Q

What are the processes that lead to the destruction of soil structure?

A

Loss of SOM, especially fresh inputs
- deserts (not much growing/decomposing)
- soil sealed by construction
- agricultural fields (most above ground biomass OM is harvested)

Rain-drop impact on bare soils

Reduction in root growth, especially in annual crops
- Produce only small roots since there is enough fertilizer to provide nutrients

Tramping by stock and machinery
- particularly in grasslands (cattle)

Reduction in soil organisms (fewer earthworms)
- due to pesticides

Loss of flocculating cations (Ca2+ and Mg2+) and replacement by dispersing cations (Na+ and K+)

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28
Q

What are some structural problems with soil and their effects?

A

Breakdown of weakened aggregates at soil surface
- Erosion removes fine particles and OM
- Surface crusting reduces seedling emergence

Formation of pan at base of plough depth and compacted subsoil
- Pan layers and compact subsoils impede root growth and water and air movement

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29
Q

What does soil mineralogy influence?

A

Influences chemical weathering, nutrient supply and buffering capacity

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30
Q

Primary minerals

A

Derived from igneous or metamorphic rocks
Mostly found in sand and silt fraction
Foundation of soil development, can make some prediction about when primary minerals are known

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31
Q

Secondary minerals

A

Inherited from parent material or formed in situ
Mostly found in the clay fraction

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32
Q

What is the difference between clay minerals and clay size fraction?

A

Clay size fraction only refers to the size of the particle (can be primary or secondary minerals)

Clay mineral only refers to secondary minerals

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33
Q

What are the primary minerals in soils?

A

Quartz (SiO2) –> most often present
Feldspar (KAlSi3O8)
Mica
Olivine
Haematite –> predominantly found in tropical regions

  • quartz, feldspar and mica make up granite
  • they all consist of silicates (apart from haematite)
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34
Q

Describe the silicon tetrahedron

A

4 oxygen atoms
1 Silicon atom
Several silicate atoms can share 1 O atom

(SiO4)^4-
Always negatively charged
Negative charge shared with adjacent Si atoms or with cations (ex. Fe3+ or Mg2+) in mineral lattice
There can be the incorporation of cations into mineral lattice: mainly Fe2+ and Mg2+

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35
Q

What does the ratio of charge to relative size determine?

A

Bonding strength (of cations) in the silicate mineral lattice

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36
Q

When energy of formation of cations in silicate minerals increase, what happens to the strength of bonding in minerals?

A

Increases as well
The larger the enery, the stronger the bond withing these minerals (more resistant to weathering)

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37
Q

Rank the energies of silicon, iron, sodium, potassium, aluminum, magnesium and calcium in increasing order

A

potassium, sodium, calcium, magnesium, iron, aluminum, silicon

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38
Q

What are some characterisics of island silicates?

A

no sharing of oxygen by Si
All negative charges are balance by Fe2+ and Mg2+
Weak bonds result in rapid chemical weathering
Sharing incorporated iron and magnesium atoms which cause the binding
Prone to weathering
ex. olivine

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39
Q

What are some characterisics of pyroxenes?

A

Single chain
2 charges per tetrahedron shared by Fe and Mg and 2 by Si
More resistant to weathering
ex. augite
(2 shared O, 2 unshared O)

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40
Q

What are some characterisics of amphiboles?

A

Double chains
2.5 charges per tetrahedron shared by Si
Only 1.5 shared by Fe and Mg
Increasing resistance to weathering
Larger proportions of oxygen shared with silicon, therefore more resistant to weathering compared to pyroxene and island silicates
ex. hornblende
a) 2 shared O, 2 unshared O
b) 3 shared O, 1 unshard O
–> depends on position in the chain

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41
Q

What are some characterisics of phyllosilicates?

A

Sheet sructure
Si-O sharing in all basal O atoms (3)
Apical O atom shared with Fe, Mg (1)
Greater resistance to weathering
a) 2 shared O, 2 unshared O
b) 3 shared O, 1 unshard O
–> depends on position in the structure

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42
Q

Quartz

A

Most prominent primary mineral in soil
Most resistant to weathering
- takes much longer to weather away and be lost, therefore remain in the soil whereas all other minerals are weathered away

3-d sharing of Si tetrahedra
- all 4 O are shared

No incorporation of Fe or Mg
- really pure, almost transparent (no colouring)

Very resistant to weathering

43
Q

Isomorphous subsitution

A

Replacement of central Si atom by Al as mineral forms from molten magma. Results in aluminosilicate minerals

Similar size but difference in valency

Isomorphous: no change in structure, what changes is the charge (valency)

Results in loss of one positive charge for each atom
- Si4+ –> Al3+ (replacement of silicon atom with aluminum atom)

Requires incorportation of cation (ex. K+, Na+, Ca2+) into mineral lattice to provide extra positive charge

44
Q

Feldspars

A

Substitution of ca. 25% of Si4+ by Al3+ and incorporation of cation (negative charge) into mineral lattice

  • Albite - Na: NaAlSi3O8
  • Anorthite - Ca: CaAl2Si2O8
  • Orthoclase - K: KAlSi3O8

K, Ca, and Na balances the charge

45
Q

Secondary minerals

A

Mostly found in the clay size fraction (only refers to size), therefore, often called clay minerals (refers to chemical composition)

Forned under conditions close to surface temperature and pressure (formed within the soils)

Resulting from weathering of primary minerals or formed in situ in soil (locally)

In soils of temperature regions alumino-silicates are most prevalent

In soils of tropical regions hydroxides of iron and aluminum are more prevalent

olivine or island silicate minerals are prone to weathering –> secondary minerals

Primary minerals are the backbone to secondary minerals

46
Q

What are some examples of clay minerals?

A

Kaoline, illite, smectite

47
Q

The aluminum octahedron

A

6 oxygen atoms
1 aluminon atom

structural network of octahedra forming an alumina sheet
can form structure networks –> aluminum sheets where aluminum atoms share oxygen atoms

48
Q

How are clay minerals structured?

A

They are composed of layers
Mutliple different sheets

Example: kaolinite –> 1:1 clay mineral
- one tetrahedral sheet with one aluminum octahedral sheet

Example: smectite –> 2:1 clay mineral
- 2 silicon tetrahedral sheets to 1 aluminum octahedral sheets

49
Q

What are some characteristics of clay minerals?

A

Large surface area
Expansion and contraction upon wetting and drying
Control cation and anion exchange capacity
Different clay minerals are formed by varying combinations of Si and Al sheets and by isomorphous subsitution
Sheets forms layered on top of each other to form these minerals
Not chemically bond but rather electrostatic force, these sheets can be subject to expansion and contraction
- electrostatic charges on clay surfaces attract water molecules

50
Q

Explain the structure of kaolinite

A

1:1 tetrahedral and octahedral sheet
one-expanding (no swelling)
Very close together
Do not share O
Strong electrostatic charge which is why it is not subject to significant expanding/contracting

51
Q

Explain the structure of smectite

A

2:1 tetrahedral and octahedral sheet
Expanding (max. swelling)
Easy for water molecules to differ in these spaces leads to expansion of minerals (sheets are pushed further away from each other)

52
Q

Explain the structure of vermiculite

A

2:1 tetrahedral and octahedral sheet
Expanding (some swelling)
Other ions such as Mg creates strong electrostatic force –> less expansion/contraction

53
Q

Explain the structure of fine-grained mica

A

2:1 tetrahedral and octahedral sheet
Non-expanding (min. swelling)
Densely compacted potassium cations which creates strong electrostatic force, therefore does not expand/shrink depending on soil water

54
Q

Explain the structure of chlorite

A

2:1 tetrahedral and octahedral sheet
Non-expanding (min. swelling)
Hydroxide creates strong electrostatic force

55
Q

What types of temperatures and precipitation patterns accelerate weathering processes/loss of cations?

A

High temperature and high precipitation
Therefore, hot and wet climates (tropical regions) –> direct formation of kaolinte, oxides of Fe and Al

56
Q

What are sesquioxides?

A

Fe and Al oxides and hydroxides

Weathering products of clay minerals that have lost all Si4+ and most other cations except Fe3+ and Al3+

Consist of modified octahedral sheets with Fe3+ or Al3+

Have no silicon tetrahedral sheets

Little or no isomorphous substitution (if so, only at the edges of the minerals)

Little cation exchange capacity

Posses covalently boound OH- ions which may cause strong absorption of certain anions like phosphate

57
Q

Gibbsite

A

Usually found in highly weathered soil

Strong bonds: exhibits partially positive charge on edges

58
Q

Give some examples of sesquioxides

A

goethite Fe(OH)3 - brown

limonite - yellow

haematite Fe2O3 - red

gibbsite Al(OH)3 - colourless

alumina Al2O3 - colourless

59
Q

Soil colour

A

Dark colours are usually indicative of high organic contents

Red colours are characteristic of soil rich in iron oxides

Blue-grey colours indicate the presence of iron in its reduced form (Fe removed under reducing conditions)

60
Q

What is used to determine colour in laboratory?

A

The Munsell soil colour chart

3-d description of colour
- hue: mixtures of red (R) and yellow (Y) (x-axis)
- value: lightness (y-axis)
- chroma: purity of strength (z-axis)

Example: 10 YR 5/2
where 10YR is the hue, 5 is the value and 2 is the chroma

61
Q

What is the importance of sesquioxides in soils?

A

Dominates soil colour where low OM content (particularly in subsoil)

Form cementation in soil when large oxide or hydroxide content - laterite in tropics

Provide anion exchange capacity

62
Q

What is a cation exchange capacity?

A

Ability to exchange cations between partciles surface and soil solution (soil solution is the liquid phase, rainwater that percolated in the soil)

Negative charge derived from clay minerals and humus

Exchange reactions are not always reversible
Released cation can either precipitate, volatilize, or strongly associate with an anion

Sources of charge are:
1) isomorphous subsitution within clay minerals
Si4+ –> Al3+; Al3+ –> Fe2+, Mg2+

2) broken bonds at edge of clay minerals
Si - O -
Al - OH -

3) dissociation of bonds at edge of humus
COO-H+ (carboxylic) or CO-H+ (phenolic)

63
Q

Cation exchange between soil particles and soil solution

A

Soil particle edges have negative charges

Cations on the particle interact with cations in soil solution –> exchange processes

Replacement of base cations with H+, e.g. from acid rain

Displacement of H+ from exchange complex by base cations, through fertilization or nutrient release from decomposing tissues

64
Q

What does binding strength of cations have to do with cation exchange capacity?

A

Some cations bind more tightly than others to the surface of colloids

The higher the charge and the smaller the hydrated radius of the cation, the more strongly it will absorb to the colloid

The relative strengths of absorption order may be altered on certain clay minerals
- ex. the high affinity for K+ ions (and the similarly sized NH4+ and Cs+ ions) of vermiculite and fine-grained micas, which attract these ions to intertetrahedral spaces

The likelihood that an absorbed cation will be displaced is also influenced by how strongly its neighboring cations are absorbed

65
Q

What is the strength of absorption for these cations decreasing order: Cs+, K+, Al3+, Li+, Na+, NH4+, Sr2+, Ca2+, Mg2+

A

Al3+ > Sr2+ > Ca2+ > Mg2+ > Cs+ > K+ = NH4+ > Na+ > Li+

66
Q

How do you measure cation exchange capacity?

A

Cations per mass of soil
Expressed as mass of cations not useful, because depends on relative mass of cations
Therefore, expressed on basis of charge characteristics of particles, as m.e./100g or cmol/kg)

Dry soil is placed in a funnel and NH4Cl solution is added
Exchangeable cations are displaced by NH4+ and washed into beaker
The solution in beaker is analyzsed for Ca, Mg, K, Al, and H

67
Q

How do you calculate cation exchange capacity?

A

Divide the mass per soil (mg/kg) by the atomic mass (it will give result in mmol)
Convert to cmol
Multiply by # charges of ion

Example: 800 mg/kg of Ca2+
atomic mass of Ca=40.078

800 mg Ca/40.078 = 20 mmol Ca
20 mmol Ca –> 2 cmol Ca
2 cmol x 2 charges = 4 cmol (+) charge

68
Q

Rank the cation exchange capacity in various soils in increasing order

A

Loamy sand, sandy loams, Fe, Al oxides
Kaolinites, silt loams
CLay loams, fine-grained micas and chlorites
Vertisols, finished compost
Smectites, Histosols, vermiculites
Soil Humus

–> organic component has largest cation exchange capacity (can buffer acid rain)

69
Q

What is the effect of soil organic carbon on cation exchange capacity?

A

When pH is very acidic, CEC does not increase a lot with an increase in C

When pH is 5.0-5.5, CEC increases a bit more with increase in C

When pH is > (8.2), CEC increases a lot with increase in C

In other words, the higher the pH, the more effect you have for the addition of 1 unit of organic carbon
- at high pH, soil with high SOC will have very high CEC compared to same soil with lower pH

70
Q

Anion exchange capacity

A

Al3+(OH)3- and Fe3+(OH)3-
Affects anions such as Cl-, NO3-, SO4(2-), PO4(3-) and organic matter (COO-) can displace OH- groups from sesquioxides

Phosphate is a limiting nutrient –> plants take it up
Phosphate run off from agriculture

71
Q

What is the influence of soil pH on soil charge in clay minerals?

A

Acid conditions: positive charge
Alkaline conditions: negative charge

more pH drops, more the net charge drops

72
Q

What is the importance of soil organic matter?

A

Helps develop structural strength through bonding of soil minerals (cementing agent)

Improves porosity and pore size distribution - increases infiltration rate, permeability and water availability (impact on soil hydrology)

Releases nutrients through decomposition

Possesses cation exchange capacity to supply nutrients to plants and store cations added as fertilizer, as well as buffering capacity.
- organic soil has a high cation exchange capacity

73
Q

What does the conversion of land use do to atmospheric CO2 levels?

A

Soil organic matter is a huge terrestrial sink for atmospheric CO2. Therefore, converting this land releases captured CO2 back into the atmosphere (SOM becomes a source of atmospheric CO2 rather than a sink)

74
Q

What is the global distrubtion of organic carbon in the upper 1 meter of the Earth’s soils?

A

There is more OC in higher latitudes and more near the equator since there is large primary productivity in the equatorial regions.

High latitudes have low temperatures (not much aboveground carbon stored). The SOM strongly increases the further north you go since there is low decomposition rates

75
Q

What does the soil organic matter content depend on?

A

Input: rate of input of plant tissues (litterfall and root death) - plant productivity

Output: rate of decomposition by soil organisms, erosion, and leaching by water

76
Q

Give some examples of inputs of organic matter to soils

A

Above-ground plant inputs: leaves, needles, and wood

Below-ground plant inputs: roots (coarse and fine)

Microfauna: bacteria and fungi

Macrofauna: arthropods and earthworms

77
Q

What is the composition of plan residues?

A

Leaf litter breaks down into 75% water and 25% dry matter. Biomolecules are added to soil via litter input. Decomposition of litter starts with shredding by some microfauna (ants, earthworms).

The types of compounds within this is 45% cellulose, 20% lignin, 18% hemicellulose, 8% protein, 5% sugar and starches, 2% fats and waxes and polyphenoids

This also breaks down into elemental composition: 42% carbon, 42% oxygen, 8% hydrogen and 8% ash

78
Q

What are some outputs of soil organic carbon?

A

Decomposition, leaching, erosion, crop harvest (agricultural fields and managed forest ecosystems), deforestation, forest fires

79
Q

What type of reaction is decomposition?

A

Oxidation

Reactants: carbon and hydrogen containing compounds

Products: CO2, H2O and energy

In a well-aerated soil, all of the organic compounds found in plant residues are subject to oxidation unless they become stabilized in the soil matrix.

80
Q

Rank the rates of decompositions of these compounds from most rapid to slowest: cellulose, crude proteins, sugars starches simple proteins, lignins phenolic compounds, fats and waxes, hemicellulose

A

1) Sugars, starches, and simple proteins
2) Crude proteins
3) Hemicellulose
4) Cellulose
5) Fats and waxes
6) Lignins and phenolic compounds

81
Q

How does cell wall impact decomposition rate?

A

Protective cell walls (make it so that they decompose much more slowly)

I.e. sugars and starches deplete very quickly and there is the accumulation of lignins and phenolic compounds since they decompoe much more slowly

82
Q

Name some factors controlling decomposition rates and completeness

A

Environment: temperature, moisture
Soil microbes: activity, composition
Litter quality: Lignin:N ratio, phenolics
Soil fauna: activity, composition

(temperature, moisture, composition of plant tissue and organic matter, nutrient content of soil, macro- and micro-organisms in soil)

83
Q

What is the product of the final stage of the decomposition process that can stabilize itself within the mineral phase in the soil?

A

Humus

84
Q

What happens to simple components in the decomposition process?

A

Sugars and proteins will mineralize very quickly and be utilized by microorganisms or plants.

When these die, they wil release biomolecules –> simple components or resistant components

–> slow decay leads to humus formation

85
Q

What is the effect of temperature on decomposition rates?

A

The warmer it gets, the larger the decomposition rate.
Howevere there is an optimum at 25-30 degrees.
Past this threshold, microorganisms can decompose less well and so decomposition rate will decrease.
Moreover, certain microorganisms can continue to decompose below 0 degrees, but the rate is much slower

Q10=increases in decomposition over 10 degree C rise

86
Q

What is the impact of water content on decomposition rate?

A

More decomposition will occur when conditions become more dry since there is more air that diffuses in the soil space (i.e. more oxygen) –> decomposition uses O to oxidize soil organic carbon

Optimum water and air content (field capacity): after field capacity has been reached, decomposition rates will decline due to a lack of water
- before optimum, increases due to increase in O
- after optimum, decreases due to lack of H2O

Fully saturated soil (anoxic conditions) can still find decomposition

87
Q

What happens to decomposition in anaerobic soils?

A

O2 is depleted when soil pores are filled with water

Without O2, aerobic organisms cannot function
Anaerobic or facultative organisms become dominant
- facultative organisms can exist in anaerobic and aerobic conditions

Products of anaerobic decomposition include organic acids, alcohols, and methane

Example: propionate + H2O – (bacteria) -> acetate + methane
- methane is a GHG

Anaerobic soils accumulate large amounts of partially decomposed OM because
- decomposition takes place much more slowly
- certain products of anaerobic metabolism are toxic to many microbes, acting as a preservative for OM

Anaerobic decomposition releases little energy. End products still contain much energy. For this reason, alcohol and methane, which as produced by anaerobic decomposition, can serve as fuel.

88
Q

What happens to leaf decomposition in forests vs. swamps?

A

Swamps have anaerobic conditions in the soil. Therefore, they take more time to decompose (more mass remains)

89
Q

What does the exponential decay constant k represent?

A

The rate at which OM decomposes
The more negative the k, the faster the decomposition

90
Q

Why do tropical regions have higher decomposition rates compared to boreal regions despite leaves being more protected?

A

k relates to environmental conditions rather than leaf composition.

91
Q

What is the impact of C:N ratios on decomposition rates?

A

THe more N soil has, the faster the decomposition rates.

C:N ratios of forest leaves are usually much higher than grasses or clover (clover is a legume and so can fix N from the atmosphere with the help of nitrogen-fixing bacteria)

Therefore, leaves that have more C and less N will decompose much slower.

Deciduous leaves, evergreen needles, roots, wood all have very high N content –> decompose quickly

92
Q

Summarize the decomposition process

A

Mechanical shredding of litter by soil fauna

Carbon compounds are enzymatically oxidized to producve CO2, water and energy

Nutrients such as nitrogen, phosphorous, and sulfur are released and/or immobolized

New compounds are synthesized by microbes as cellular constituents or as breakdown products

Some of the original plant compounds, their breakdown products and microbial compounds become physically or chemically protected from further microbial decay via interactions with the soil minerals

93
Q

Describe the different levels of organic substances in the soil relative to time

A

Fresh residues added: mainly compounds in the organic tissue and old soil humus

As time goes on, there is a constant decrease in organic tissue. There is a slight dip in humus level during the time that there is an increase in microbial biomass (this dip is called the priming effect). As microbial biomass decreases, there is an increase in soil humus.

In the end, there is new soil humus (more than before). C from residues decomposed or stabilized.

94
Q

What happens to global soil content related to temp. and precipitation?

A

the higher the temp and the lower the precipitation, the higher the SOM

??? check this ???

95
Q

How can OM be lost?

A

Forest fires: can burn much of forest floor.
- Leads to a huge loss of soil organic carbon. SOM will start to recover after the fire.
- Slow build-up of OM in post-fire recovery of vegetation. Important in boreal forests.

Agriculture:
- high plant productivity but much of OM removed in crops and frequent tillage and fertilizer application can speed rate of decomposition of OM.
- Large part of SOM is taken away from the site.
- Decomposition processes due to tillage (soil is turned over), soil organic carbon is exposed to the atmosphere
- loss of soil quality

96
Q

Rank from highest loss to lowest the changes in soil organic carbon (%) associated with change in land use of:
- forest, grassland, and savanna, to cropping, grazing and plantation

A

forest: cropping, plantation, grazing
grassland: grazing, cropping
savanna: cropping, grazing, plantation

97
Q

Compare the historical view of soil organic carbon to the emerging understanding

A

Historical view:
- fresh plant litter (leaves) –> molecular structure determines timescale of persistence (condensation reactions –> creawtion of new stable compounds)

Emerging theory: integrative view of env. conditions as well as soil
- fresh plant litter (leaves, stems, roots, and rhizosphere), fire residues
- Physical disconnections (from enzymes, decomposers, e-acceptors)
- Deep soil carbon: age of carbon reflects timescale of process. Rapid destabilization possible with change in environmental conditions
- Freezing/thawing
- Microbial products

98
Q

What is black carbon?

A

The byproduct of the chemical-thermal conversion of carbon-containing material

Variable in properties, depending on origin and formation, temperature and duration

99
Q

What is biochar?

A

Refers to black carbon that is produced as a vehicle of carbon sequestration from biomass

100
Q

Is all biochar black carbon and vice-versa?

A

Biochar is black carbon, but not all black carbon is biochar

101
Q

What is pyrolysis?

A

The process of thermochemical decomposition of OM at elevated temperatures in the absence of oxygen.

102
Q

How is black carbon/biochar produced?

A

Produced naturally through wildfires in grasslands, forests, etc.
By slash and burn activities and deliberately by humans

103
Q

What are the effects of biochar on environmental and crop yields/fertility?

A

pH high (generally 6-10): generally raises soil pH

May reduce nutrient losses through absorption/immobilization

High surface area and reactivity may lead to heavy metal, phosphorus and antibiotic sorption

May reduce emission of N2O

Increases water holding capacity of soils

Not all soils will beenfit from biochar applications; putting biochar on degraded or sandy soils where productivity is limited by low nutrient or water holding capacity is likely to be far more beneficial than adding biochar on highly productive soils.

104
Q

What are black earth soils (terra preta)?

A

Most likely created by pre-Columbian indigenous farmers from 500-2500 years B.P. and abandoned after the invasion of Europeans (argued that abandonment associated with decline derived from European diseases)

Contain 150 g C/kg (15%) soil in comparison to the surrounding soils with 20-30 g C/kg (2-3%) soil.

Characterized by high P contents reaching 200-400 mg P/kg, and higher cation exchange capacity, pH and base saturation than surrounding soils.

These soils are therefore highly fertile. Fallows on the Amazonian Dark Earths can be as short as 6 months, whereas periods on Oxisols are usually 8-10 years long.

GEOG 305 (3 decks)

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