lecture 9 Flashcards
O2 consumption is related to what?
- Root respiration
- Decomposition of organic matter by microorganisms
What are the factors that influence air content of soils?
- Water movement
- Water content
- Porosity
- Soil structure
- Chemistry biology
- Soil texture
In soils, where is CO2 derived from? What do the emissions rate depend on?
About half of CO2 emissions are derived from root respiration and decomposition.
CO2 emission rates depend mostly on temperature, moisture, and nutrient supply.
What is the chemical composition of respiration?
glucose + oxygen (substrate) –> CO2 + water (waste products) + energy
What happens when soils are water logged?
- Impeded soil aeration e.g. due to water logging, leads to a depletion of O2 and anaerobic condition
- The result is an inhibition of plant growth that requires O2 for root respiration (mesophytes)
- Some specialized plants can transfer O2 from the atmosphere via leaves and stems to their roots (hydrophytes) aka they can transpire during water logged conditions
- Anaerobic microorganisms becomes dominant e.g. methanogens
What is the chemical formula of methanogenesis?
CO2 + 4H2 –> CH4 + 2H2O + energy
What kind of reactions are respiration and methanogenesis?
reduction-oxidation (redox) reactions
OIL RIG
Oxidation is loss of electrons
Reductionis gain of electrons
In what way does soil respiration depend on the diffusion of oxygen into the soil profile? Under what condition is it ideal?
Oxygen is required for soil respiration. Oxygen diffuses much faster through air than through water.
After a dry period, oxygen is located in the pore spaces of a soil. The pore spaces can be rapidly depleted after rainfall because the water will push out the soil.
Therefore, oxygen from the atmosphere will take longer to diffuse in saturated conditions.
What is redox potential?
The potential for electron transfer from one substance to another
Would pH increase or decrease in a soil when it experiences methanogenesis?
During methanogenesis, the reduction of CO2 occurs rather than the oxidation of a carbon compound.
This causes the pH to increase. The consumption of the H+ ions increases the pH.
Explain how the redox potential in soils changes as aeration is reduced (aka more saturated).
In a well-aerated, non saturated soil, there is plenty of O2, so the redox potential is high.
As aeration is reduced and gaseous O2 is depleted, the redox potential declines. The redox potential is particularly low in organic matter-rich soils that are flooded under warm conditions.
When the soil is fully saturated, no more O2 is available, the there is no more redox potential. Only anaerobic microorganisms survive, which use substances other than O2 as electron acceptors for their metabolism.
Explain the process of oxidation of Iron. What impact does this have on pH? What are the impacts of this oxidation on the soil’s appearance?
In this oxidation, Fe2+ is an electron donor. Fe2+ loses an electron to become Fe3+ and forms H+ ions by hydrolyzing H2O.
2FeO (2+) + 2H2) –> <– 2feOOH (3+) + 2H+ + 2e
The H+ ions lower the soil pH.
If iron becomes reduced, the soil can become a grey/blue colour.
What is the order of the redox ladder, starting with oxygen?
- O2 –> H2O (respiration)
- NO3- –> N2O and N2 (denitrification)
- MnO2 –. Mn2+ (manganese reduction)
- Fe(OH)3 –> Fe3+ (iron reduction)
- SO4- –> H2S (sulfate reduction)
- CO2 –> CH4 (methanogenisis)
What are the reasons for poor plant growth in waterlogged soils across the redox sequences?
- No more O2 for respiration
- Increased Mn and Fe solubility and toxicity if concentration is too high
- Loss of NO3- (nitrate) by denitrification: N deficiency
- SO42- reduction to sulfides e.g. H2S which may be toxic to plant roots
