Midterm Lectures 6-10 Flashcards
(125 cards)
1
Q
How are micropores created?
A
Microaggregates do not pack well together –> creates micropores and takes up large amounts of space in the soil.
2
Q
Is the pore space higher for clay or sand?
A
Clay
3
Q
What is soil water?
A
Water occupies pore space along with air
In most soils, pore space is 50-60% of volume
Important are amount of water in the soil as well as the chemical composition of the soil solution
4
Q
How do you measure soil water content?
A
Gravimetric: measure the mass of water within a certain amount of soil
- Weigh fresh and after drying at 105 degrees C. - Limited utility (because total amount of water fluctuates substantially
Volumetric: requires knowledge of bulk density of soil to convert gravimetric to volumentric values
5
Q
Why is volumetric water content useful?
A
It compares with point measurements such as precipitation, evapotranspiration and water storage capacity
6
Q
What is bulk density?
A
The mass of a unit volume of dry soil. The volume includes both solids and pores (basically the density of the material)
Lower bulk density: lower weight, more pore space
Higher bulk density: higher weight, less pore space
7
Q
How do you measure bulk density?
A
Can only be done in the field
A soil auger with an inner cylinder is driven into the soil
The inner cylinder with known volume containing an undisturbed soil core is then removed
The weight of this soil core is then determined after drying in an oven
The difference of these two weights provides you with the volumetric water content.
8
Q
What is peat?
A
Organic soil that has been decomposed for thousands of years and leads to the accumulation of OM
9
Q
What has the largest bulk density between OM and quartz mineral?
A
Quartz mineral (OM very low bulk density)
10
Q
What is the H2O molecule made up of?
A
Polar molecule with partial negative and partial positive side to it (due to the angle of the molecule)
Oxygen nucleus, with two hydrogen nucleuses
11
Q
What is adhesion?
A
Electrostatic binding of positively charged side of the molecule. H on the negative side.
Can bind to SOM (since it has a lot of negative charges on the surface)
12
Q
What is cohesion?
A
The binding of H atoms in the water to the O of a neighbouring molecule.
Forms network of connected water molecules that can change in size and shape.
Ex. mosquitoes on water (the cohesion forces of the water can support the weight of the mosquito)
Ex. meniscus (water molecules climb on the surface due to cohesion)
13
Q
Hydrophobic surface
A
Waxy surface
Pushes down water column below the water table, the droplet will not spread out but rather try to keep its surface as small as possible
Ex. plant leaves in the rainforest (since it is so most and humid in the tropics, strong waxy surfaces avoid infestation by fungi and mold)
14
Q
Hydrophilic surface
A
Water droplet will spread as large as possible on the surface
15
Q
How is the height of rise on hydrophilic surfaces related to the capillary tube radius?
A
The height of rise is inversely proportional to the capillary tube radius
16
Q
How is pore size related to the height of rise on hydrophilic surfaces?
A
The pore sizes in soil is inversely proportional to the height of rise since the finer the texture, the smaller the pores, the higher the capillary rise
17
Q
What are 4 types of soil water potential?
A
Gravitational potential
Hydrostatic potential
Matric potential
Osmotic potential
18
Q
What is gravitational potential?
A
Due to differences in elevation of soil water relative to reference pool. Used to calculate movement in saturated soils through hydraulic conductivity and head.
The higher a body of water is above the relative reference pool, the higher the gravitational potential.
19
Q
What is hydrostatic potential?
A
Due to the weight of overlying water in saturated soils (positive pressure)
20
Q
What is matric potential?
A
Measure of bonding strength between soil particles and soil water. Difference in potential due to attractive forces between soil water and solids and pure water (negative pressure).
Relates to the plant availability of water and capillary rise
Even more negative than the osmotic potential
21
Q
What is osmotic potential?
A
Associated with solutes in soil water (ex. NaCl)
Important in reducing effective availability of water to plants in saline soils
Negative potential
22
Q
What is tortuosity?
A
The process by which water will not take straight paths in soil, rather it will winds along particles
23
Q
What is the matric soil water potential of a saturated soil?
A
Matric potential is zero. 0 MPa soil water potential
24
Q
What is the matric soil water potential of a soil at field capacity?
A
Point at which removal from soil by drainage is very slow (usually 2 days after rain) –> until soil will hold water against gravity (water will no longer percolate)
Equilibrium between gravitational forces and bonding strength of solids for soil water. 0.01-0.03 MPa soil water potential (0.1-0.3 bars of atmospheres)
25
What is the matric soil water potential when a soil is at a wilting point?
Point at which plant uptake of water for transpiration is severely reduced, resulting in wilting of plant.
1.5 MPa soil water potential (15 bars of atmospheres)
26
What is the matric soil water potential at hygroscopic coefficient?
No further drying in air, representing balance between evaporative forces and bonding strength of solids-water.
There is a thin film of water on every surface (binds a tiny amount of water molecules). Even if you dry soil as much as you can, there will still be tiny amount of water.
3.1 MPa soil water potential (31 bars of atmospheres)
27
What soil has a lower available water capacity?
Clay soils have lower available water capacity compared to silt, loam, and sandy soils even though they have a higher pore space
WHY??
28
Do all soils with the same soil moisture potential have the same moisture content?
No, moisture content also depends on soil texture
29
Is soil moisture content higher in silt of sand?
Silt
30
How is bulk density related to soil texture?
Bulk density decreases with texture (finer --> decreases)
31
How is porosity related to soil texture?
Porosity increases with texture (finer --> increases)
32
How is available water capacity related to available water for plants?
There is an optimum in the range of silt loam --> most available water for plants
Even though clay soils can contain a large amount of water, it is not necessarily available for plants. The increasing matric potential at a certain point outcompetes the amount of water. It holds on the water too strongly on the surface.
33
How is volumetric water content related to soil texture?
Increases with texture (sat. field capacity and wilting point)
the finer the texture --> increases
34
Why is the wilting point and field capacity of sand low?
Small surface area and therefore not much water will be held back by soil surface.
Field capacity if also very low (since water drains through quickly)
35
Why is the field capacity and wilting point for clay high?
Field capacity is high since water drains very slowly.
However, large amount of surface makes water unavailable to plants since it will stick to the surface.
36
How do you determine available water with a graph that has field capacity / wilting point?
Available water = field capacity - wilting point
Area in between the two plots (graph slide 14 lect. 6)
37
How is OM connected to field capacity and permanent wilting point and available water?
The more OM, the larger the PWP and FC
High OM = high amount of available water
38
What is the soil water movement in saturated conditions?
Fast movement in large pore spaces (sands)
Slow in small pore spaces (clays)
39
What is the soil water movement in unsaturated conditions?
Affected by volumetric water content and connectedness between water films along pore walls
40
What is Darcy's law and what is it used for?
Saturated flow through soils
Useful to know about groundwater recharge
Predicts floodings, how quickly does rainwater percolate and reach water table
Q/T = AKs(change of water pot./L)
where
Q=qty of water
T=time
A=cross sectional area through which water flows
Ks=saturated hydraulic conductivity (cm/s)
delta=change of water potential
L=length of the pathway
41
What is saturated hydraulic conductivity (Ks) important for?
Irrigation
Sanitary landfill
Cover material
Waster water storage
Septic tank drain
42
What are the factors that influence Ks
Pore size (sandy soils > clayey soils)
Packing of particles (depends on shape of soil particles)
- soil compaction creates a situation of tight packing over loose packing
Soil structure (aggregation)
Biopores (root channels due to earthworms) --> these can create preferential flow paths
Preferential flow
43
What is the moisture movement in unsaturated soils influenced by?
Water content, porosity, connectedness and frictional drag from pore walls
44
How does connectedness between water films along pore walls influence moisture movement in unsaturated soils?
Water is no longer connected in sandy soils --> water can still flow in silt since it it still connected
45
What is an acid?
Any substance which increases the hydrogen ion concentration in the soil solution
H+
46
What is hydrolysis of water
Ionization of water
H2O --> OH- + H+
where OH- is the base and H+ is the acid
In pure water, the amounts of H+ and OH- are equal
47
What is pH?
THe ion product of the concentrations of H+ and OH- ions is a constant (Kw) which is known to be 10^-14 mol^2L^-2
The very small concentrations of H+ and OH- ions is expressed by using the negative logarithm of the H+ ion concentration termed pH
If the H+ concentration in an acid medium is 10^-5 mol L^-1, the pH is 5
If it is 10^-9, in an alkaline medium, the pH is 9
Whether a soil is acid, neutral, or alkaline is determined by the comparative concentrations of H+ and OH- ions.
48
What is the importance of soil pH?
Determines nutrient availability, in particular phosphorus
Influences cation exchange capacity
Influences earth worm activity and other fauna in soils
Strong influence on the microbial community in soil
Low soil pH can lead to aluminum toxicity for plants
49
How is nutrient availability affected by soil pH?
P is less available in low and high pH levels (most available at around pH of 6-7)
- P will react with Fe and Mn in low pH environments
- P will reaction with Ca and Mg in high pH environments
N, K, S is increasingly available as pH increases
Al available at low pH, decreases as pH increases
50
How is cation exchange capacity impacted by soil pH?
More acidity --> less base cations are available for plants
The addition of H+ will make Ca, K, Na, and Mg leach out of the soil (H+ will exchange with them)
51
How is earth worm activity and other fauna in soils affected by soil pH
Earth worms are important in building soil structure, erosion and health
Earthworm survival is pH dependent (very sensitive to soil acidity --> survival rates drop in soils with pH 3 and lower)
Same goes for microbial community in soil (neutral pH is optimal)
52
How is aluminum toxicity related to soil pH?
Al will become mobilized at pH lower than 5
Al concentration will increase starting at pH 5 and lower
Some grass species do not respond strongly to increase in Al, whereas others are highly sensitive to Al concentrations in soils
53
What are some sources of hydrogen ions?
Carbonic acid (H2CO3)
Organic acids
Accumulation of OM
Oxidation of nitrogen and sulfur
Acid rain
Plant uptake of cations
54
What is the impact of carbonic acid?
CO2 + H2O --> H2CO3
H2CO2 <--> HCO3- + H+ + energy
The formation of H2CO3 and the subsequent dissociation of its H+ ions occurs when CO2 dissolves in water
Root respiration and the decomposition SOM by microorganisms produce high levels of CO2 in soil air
CO2 in atmosphere will dissolve in water, will form bicarbonate and then later will dissociated and produce H+ (associated to ocean acidification)
The more rain you have the more H+ atoms will be introduced in the soil
55
What is the impact of organic acids?
OM + O2 + H2O <--> strong organic acids <--> RCOO- + H+ + energy
Microbes generate organic acids as they break down OM in the soil
Some of low molecular weight organic acids, such as citric or malic acids, that only weakly dissociate
Others are more complex and stronger acids, such as the carboxylic and phenolic acid groups in humus.
56
How does the accumulation of OM affect H+ levels?
OM forms soluble complexes with non-acid nutrient cations such as Ca2+ and Mg2+, thus facilitating the loss of these cations by leaching
OM is a source of H+ ions because it contains numerous acid functional groups from which these ions can dissociated
57
How does the oxidation of nitrogen affect H+ levels?
NH4+ + 2O2 --> H2O + H+ + dissociated nitric acid
Oxidation reactions generally produce H+ ions, reduction reactions, tend to consume H+ ions
Ammonium from OM or fertilizers is converted to nitrate by microbial oxidation
This oxidation, termed nitrification, releases two H+ ions for each NH4+ ion oxidized.
58
How does acid rain affect H+ levels?
As raindrops fall through unpolluted air, they dissolved CO2 and from enough carbonic acid to lower the pH of the water from 7.0 to 5.6
59
How does plant uptake of cations affect H+ levels?
Uptake of cations balanced by release of H+ ions from root have an acidifying effect
Uptake of cations balanced by uptake of anions -- no effect on pH
60
How does presence of aluminum affect H+ levels?
Aluminum is toxic and has the ability to hydrolyze H2O and reacts with resulting OH-
Ex. Al3+ + H2O --> Al(OH)2+ + H+
One aluminum can produce 3 H+ ions!
When Al is high in your soil --> not a good sign
61
What is active acidity?
Defined by the H+ ion activity in the soil solution
This pool is very small
Extremely important, determines the solubility of many substances
62
What is exchangeable acidity?
Associated with exchangeable aluminum and hydrogen ions
This pool is large
Ions can be released into the soil solution by cation exchange
63
What is residual acidity?
Associated with hydrogen and aluminum ions that are bound in nonexchangeable forms by OM and clays
The residual acidity is commonly far greater than either the active or exchangeable acidity
64
Where is the world do we see acidic soils?
Boreal forests --> needles of coniferous trees (waxy surface decomposes more slowly)
Tundra --> cold temp., lower evaporation, slow primary production and slow rate of acc. of OM
Areas with high precipitation (tropics)
Australia used to be a location with a lot of precipitatation --> lead to the deep leaching and acc. of iron oxides in the soil
Thin red band in US west --> mountain range (precipitation)
65
How do we measure soil pH in the lab?
Bring soil into suspension with pure water or 0.1M CaCl2 solution (use calcium chloride over water because salts attached to soil will diffuse into the solution)
The difference between H+ ions in the soil suspension and in the glass electrode gives rise to an electrometric potential
(this is an approximation since we are measuring a hydrological system rather than the soil itself)
66
How is the fauna category broken down?
Macro: mammals, reptiles, insects, earthworms
Micro: nematodes, protozoa, rotifers
67
What is included in the flora category?
Plant roots, algae, fungi, actinomycetes (filamentous bacteria), bacteria
68
How are organisms classified?
Food, oxygen, energy source
69
How are organisms classified based on food?
Herbivores: eat living plants (insects, mammals, reptiles)
Detrivores: eat dead tissues (fungi, bacteria)
Predators: eat other animals (insects, mammals, reptiles)
70
How are organisms classified based on O2 demand?
Aerobic: active in O2 rich environments, use free oxygen for metabolism (using O as an electron acceptor for their metabolism)
Anaerobic: active in O2 poor environments, use of other electron acceptors (ec. NO3- or SO4(2-))
- Can also be in oxygen rich environments, however will not have a lot of activity (will be outcompeted by aerobic organisms)
71
How are organisms classified based on sources of energy and carbon?
Autotrophic: dependent on inorganic sources of energy (fixes CO2)
Heterorophic: dependent on organic matter (uses organic C)
Phototropic: dependent on solar energy (light)
Chemotrophic: dependent on chemical sources of energy (chemical)
72
Autotrophs
Produce complex organic compounds from carbon dioxide using energy from light
simple inorganic molecule + H2O --> complex organic compound + O2
Primary producers (plants, algae, cyanobacteria) - base of the food chain (produce compounds for other organisms to live on)
73
Heterotrophs
Derive energy from consumption of complex organic compounds produced by autotrophs
Autotrophs store energy from the sun in carbon compounds. Heterogrophs consume these complex carbon compounds for energy
74
Aerobic heterotrophs
Live in high-oxygen environments and consume organic compounds for energy
Obtain the energy stored in complex organic compounds by combining them with oxygen
C6H12O6 + O = energy
75
What is aerobic respiration
glucose (electron donor) + oxygen (electron acceptor) --> carbon dioxide (electron poor) + water (electron rich)
2880 kJ of energy is produced
Aerobic respiration is very efficient, yielding high amounts of energy
76
Anaerobic heterotrophs
Live in low-oxygen environments and consume organic compounds for energy
Can use energy stored in complex carbon compounds in the absence of free oxygen
The energy is obtained by exchanging electrons with elements other than oxygen (nitrogen, sulfur and iron)
77
Anaerobic respiration
glucose (electron donor) + nitrate (lectron acceptor) + water --> bicarbonate (electron poor) + ammonium (electron rich)
1796 kJ of energy is produced
Anaerobic respiration uses electron acceptors other than O2. Less efficient compared to aerobic respiration
78
Why do anaerobic organisms do not thrive in O rich environments?
Anaerobic respiration is less efficient and produces less energy. THey compete with aerobic organisms who can derive so much more energy.
79
What has an important impact on the global distribution of soil organisms?
soil pH and C:N ratios in soil
80
Where do we find a high presence of arthropods?
Forested soils with low pH and high C:N ratios
When pH increases and soil C:N ratios decrease, arthropods presence diminishes
81
Where do we find high presence of nematodes?
Forests soils with high pH and low C:N ratios
82
Where do we find the largest relative biomass of macrofauna, mesofauna and microfauna?
Macrofauna: tropics
Mesofauna: temperate forest
Microfauna: tundra/polar deserts (more north)
83
What are the major micro-organisms?
Bacteria: most common. Great variety and metabolic pathways, important in chemical decomposition of OM and chemical transformations
- outcompetes fungi in high pH environments
Algae: both terrestrial and aquatic forms, important because of ability to fix atmospheric N2 into organic and inorganic forms, mainly close to surface (dependent on photosynthesis)
Protozoa: parasites and predators, regulating microbial pop.
Fungi: comprising filaments branching through soil, displace bacteria in acidic and nutrient poor soils, decomposes recalcitrant tissues such as wood
- predominant in acidic and nutrient poor since they are very efficient decomposers (outcompetes bacteria)
84
What are some examples of soil macro-organisms?
earthworms, termites, nematodes, mites, ants, etc.
85
What is the effect of earthworms on soil properties?
Ingest and excrete large amounts of topsoil
- can churn over the top 15 cm of soil once every 15 years
Affect soil properties, both soil structure and soil nutrient availability, by mixing of organic and mineral fractions and stimulating decomposition of OM
- paths form micro and macro-aggregates (formation of soil structure)
- homogenizes soil matrix --> better conditions of availability of water/nutrients
86
Where do we find the highest density and biomass of earthworms?
Temperate pastures
- a lot of OM that earthworms can churn over
87
How does land use impact soil consumption by earthworms?
Pastures have cows that compact soil --> make it more difficult for earthworms to churn over soil
88
How do earthworms impact soil chemistry?
Exchangeable Ca in forest soil will increase (not so much K and P --> different to arable soils)
Exchanged K and P will increase in arable soils
Not much change in pH
89
Are earthworms always beneficial to all ecosystems?
No --> invasive earthworms in Noth America
Organic soils in NA have thick and stratified littler layer --> decomposed layer has not been mixed in soil profile (very loose)
Introduction of earthworms --> churn this layer and lose the thick organic layer which is negative for specie who thrive under this litter layer
- This loose, thick litter layer is essential habitat for native plants and animals
- roots become exposed, organic horizon is much smaller (mixture is much for homogenized)
90
What happens to the organic horizon (O), litter horizon (L), humus layer (H) and fibrous material (F) with the introduction of invasive earthworms in the coniferous-deciduous forest?
F and H layer is completely lost
L layer is diminished
Eluviated mineral layer becomes mixed into a mineral-humus layer
91
How do termites modify OM and nutrient distribution?
Special channels where they transfer plants, food and litter used to solidy their mound
Mounds of constructed in a way that they retain water --> larger soil moisture leads to N and P accumulation
Colonize fungi they use as a food source
Ventilation systems - important in semi-arid climates
Soil types becomes more sandy in a clay dominated soil
During the dry season, these mounds can be the location where trees grow since there is a greater availability of nutrients and water
- abandoned termine mounts can become islands of high soil fertility
91
How do termites modify OM and nutrient distribution?
Special channels where they transfer plants, food and litter used to solidy their mound
Mounds of constructed in a way that they retain water --> larger soil moisture leads to N and P accumulation
Colonize fungi they use as a food source
Ventilation systems - important in semi-arid climates
Soil types becomes more sandy in a clay dominated soil
During the dry season, these mounds can be the location where trees grow since there is a greater availability of nutrients and water
- abandoned termine mounts can become islands of high soil fertility
92
What are the direct positive effects of root exudates in the rhizosphere?
Stimulates mycorrhizal infection
Induces nodule formation
Detoxidies rhizotoxic metals (Al)
Increases nutrient availability
Improves soil water holding capacity
Surpresses pathogens
93
What are the indirect positive effects of root exudates in the rhizosphere?
Supports associative N2 fixation and N transfer to the plant
Induces plant hormone and vitamin productino by microbes which enhances plant growth
94
What are the direct negative effects of root exudates in the rhizosphere?
Induces fungal pathogen growth
Attracts root feeding nematodes
95
What are the indirect negative effects of root exudates in the rhizosphere?
Induces immobilization of nutrients making them less plant available
Induces microbial phytotoxin production
96
What is the rhizosphere's effect on micro-variability in density of organisms around roots?
Bacteria found a lot more around roots
Fungi as well
Protozoa and algae root: soil ratio is almost equal
97
Mycorrhizal infection of roots
Symbiosis
Important since the fungi supplies the plant with P and N in nutrient limiting conditions
Plants, in turn, can relocate carbohydrates to the fungi (which gives energy to fungi) --> due to photosynthesis
98
What does the mycorrhizal sheath around roots do for the plant?
Surface area of plant root is increased a lot
99
Ectomycorrhizae
Does not infest the cells, does not really penetrate the host cell walls, causes less damage to the plants
100
Endomycorrhizae
Penetrates plant cell walls
101
Mycorrhizae
Fungal: plant associatinos occur in almost all plant species, and number decreases as soil fertility increases
Plant supplies 10-30% of photosynthate to fungi
Fungi increases nutrient scavenging volume of soil, increase nutrient uptake by plant, especially N and P, allows organic forms of nutrients to be taken up
Fungi produce glomalin, a protein which is stable in soils and leads to improved binding of micro-aggregates
- thus better soil structure, infiltration, root penetration, less erosion, etc.
102
What are the positive effects of mycorrhizae in agriculture?
Enhanced nutrient uptake (in particular P --> less P needs to be applied as fertilizer)
Water supply (lower need for irrigation)
Pest control
Soil stabilisation (helps with waater holding capacity and water infiltration, helps reduce erosion)
103
How can we promote mycorrhiza in agriculture?
Organic farming (no pesticides)
Cultivation of crops which develop mycorrhiza (legumes, corn, potatoes, etc.)
- canola and sugar beet do not undergo this symbiosis --> fend off fungi
Continuous soil cover
Mild soil tillage practices (strong tillage is not good for fungi growth)
104
What is the importance of soil organisms and OM?
Helps develop structural strength through bonding
Improves porosity and pore size distribution
- increases infiltration rate, permeability and water availability
Releases nutrients through decomposition
Posses catino exchange capacity to supply nutrients to plants and store cations added as fertilizer, as well as buffering capacity (particularly earthworms)
Genetic resources --> antibiotics
- large diversity in soil organisms means high genetic diversity which is a huge resurce in production of antibiotics to fend off resistant pathogens
Diversity --> function redundancy --> resilience
- if one organism dies out, the other organism can still maintain the function (function is not lost)
- more resilient to outside influences such as acid rain, climate change, changes in species composition and soil moisture patterns
105
What are some factors that influence air content of soil?
soil texture, chemistry, biology, soil structure, porosity, water content and movement
106
What does soil aeration do?
Good soil aeration will prevent accumulation of toxic gases (CO2 and methane)
Exchange of O and CO2 between the atmosphere and the soil matrix
Microorganisms need O for respiration --> soil needs to be well ventilated
Microorganisms produce CO2 in soil and it goes into the atmosphere
Enrichment of CO2 in the soil copmared to the atmosphere
Depletion of O2 in the soil compared to the atmosphere
107
Rates of O2 consumption and CO2 emission from soils
O2 consumption related to root respiration and decomposition of OM by microbes and soil animals.
In many soils, about half of CO2 emission derived from each of these sources
CO2 emission rates
Effect on soil air composition
Land use
108
Where are the rates of soil respiration and NEP the highest?
Tropical biomes
- soil respiration and NEP is high, a lot of material is returned to soil that can be respired
Tundra is the lowest
Tipping point is boreal and temperate forests
109
What is respiration?
Chemical reactions in all living cells
C6H12O6 + 6O2 --> 6 CO2 + 6 H2O + energy
glucose + oxygen --> carbon dioxide + water + energy (usually stored as ATP)
CO2 and H2O --> waste products
110
What does diffusion of oxygen into soil profile depends on?
Pore space and the amount of water in the soil (saturated soil --> diffusion is much more slow)
O2 diffuses 10,000 times faster through air than water
After a heavy rain, amount of O in soil is much lower
111
What is the impact of iron mobilization?
Iron can remain in soil in oxidized from (Fe3+)
Fe3+ gets mobilized and turns into Fe2+
Fe2+ can be leached out and carried away to groundwater
112
What colours does a iron poor soil matrix look like?
blue-grey (Fe depleted matrix)
Sometimes we see brown Fe-rich mottles and root tubule (tubules can be due to roots)
113
Oxidation reduction reaction in soils REDOX
Oxidation is loss of electrons
Reduction is gain of electrons (becomes more negative)
OIL RIG
114
Explain oxidation
2FeO (iron 2) + 2H2O <--> 2FeOOH (iron 3) + 2H+ 2 e-
--> this is an example of an oxidation with iron2 as an electron donor
As the reaction proceeds, each Fe2 loses an electron to become Fe3 and forms H+ ions by hydrolyzing H2O. These H+ ions lower the pH
However, electrons cannot float freely in the soil solution, there needs to be an electron acceptor --> REDUCTION
115
Explain reduction
1/2 O2 + 2H+ + 2 e- <--> H2O
Oxygen rapidly accepts electrons
Aerobic respiration requires O2 to accept electrons as organisms oxidize organic carbon to provide energy for life
Note that O and O2 has zero charge but a charge of -2 H2O
116
What is the overall effect of REDOX reaction?
2 FeO + 1/2 O2 + H2O <--> 2 FeOOH
The donation and acceptance of electrons and H+ ions on each side of the equations have balanced each other and therefore do not appear in the combined reaction
117
What is the redox potential?
The potential for electron transfer from one substance to another (Eh)
Eh is expressed in volts or millivolts
When two redox couples react, the reduced substance of the couple whose Eh is mre negative donates electrons to the oxidized substance of the couple whose Eh is more positive (refer to redox tower)
The further an electron drops, the greater the redox potential, the greater energy released in net reaction
118
Redox potential in soils
In well-aerated soil with plenty of gaseous O2, the Eh is in the range of 0.4-0.7 V
As aeration is reduced and gaseous O2 is depleted, the Eh declines to about 0.3-0.35 V
If organic matter-rich soils are flooded uner warm conditions, Eh values as low as -0.3V can be found
With no O2 available, only anaerobic microorganisms can survive. They must use subtances other than O2 as electron acceptors for their metabolism.
119
Explain the sulfur cycle in waterlogged soils
Input of sulfate into water, sulfate can be taken up my microorganisms and can be buried into soil, can be dissolved by soil minerals
Sulfates will be directly taken up by plants, leached out, or taken up by clay minerals, or reduced to hydrogen sulfite --> gas will diffuse out into the atmosphere
Sulfe oxidation happens in oxidized layer of soil vs. sulfate reduction happens in reduced layer of soil (beneath oxidized layer)
119
Explain the sulfur cycle in waterlogged soils
Input of sulfate into water, sulfate can be taken up my microorganisms and can be buried into soil, can be dissolved by soil minerals
Sulfates will be directly taken up by plants, leached out, or taken up by clay minerals, or reduced to hydrogen sulfite --> gas will diffuse out into the atmosphere
Sulfe oxidation happens in oxidized layer of soil vs. sulfate reduction happens in reduced layer of soil (beneath oxidized layer)
120
Explain the nitrogen cycle under waterlogged conditions
N2 --> nitrogen fixing bacteria convert is to ammonium
- O2 diffuses from water into oxidized soil layer
NH4+ --> NO2- (nitrite) --> NO3- (nitrate)
- known as nitrification, happens in oxidized soil layer
nitrate leaches down to reduced soil layer --> nitrites --> nitrogen (nitrous oxide)
- denitrification happens in reduced soil layer
- losses back into the water as N2 and N2O
121
Why do most plants grow poorly in waterlogged soils?
No more O2 for respiration
Increased CO2 concentration in soil air
- may be toxic to plant roots
Redox sequences
- Increased Mn and Fe solubility and toxicity
Loss of NO3- by denitrification --> N deficiency
- SO4(2-) reduction to sulphides (H2S) which may be toxic to plant roots
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Wetland soils
Ecosystems that are transitional between land and water
Cover approx. 14% of the world's ice-free land
Most wetland losses occured as farmers used articifial drainage to convert them into cropland
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What are some ecosystem services that wetlands provide?
Species habitat
Water filtration (especially N and P from agricultural runoff)
Flooding reduction
Shoreline protection
Commercial and activity
Natural products (fish, blueberries, etc.)
