lecture 5 Flashcards
What is the importance of soil organic matter in soils?
- Helps develop structural strength through bonding
- Improves porosity and pore size distribution, which increases infiltration rate, permeability and water availability
- Releases nutrients through decomposition
- Possesses cation exchange capacity to supply nutrients to plants and store cations added as fertilizer, as well as buffering capacity
What does soil organic matter content depend on? What’s missing?
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
The amount of carbon that is stored, determines whether the carbon is stable or labile.
What are the inputs of organic matter to soils?
- Above-ground plant inputs: leaves, needles, wood
- Below-ground plant inputs: roots (coarse and fine)
- Microfauna: bacteria and fungi
- Macrofaune: anthropods and earthworms
What are the outputs of soil organic carbon?
- Decomposition (root respiration and microbial respiration)
- Leaching
- Erosion
- Crop harvest
- Deforestation
- Forest fires
What happens in well-aerated soil?
In well-aerated soil, all of the organic compounds found in plant residues are subject to oxidation.
Decomposition is an oxidation.
What are the controls on the rate of organic matter decomposition in soils?
- Composition of plant tissue and organic matter
- Temperature
- Moisture
- Nutrient content of soil
- Macro and micro organisms in soil
What are the elements that go trough the fastest to lowest decomposition?
- Sugars, starches, simple proteins (rapid, rich in nitrogen, phosphorus, all nutrients required for microbial growth)
- Crude proteins
- Hemicellulose
- Fats and waxes
- Lignins and phenolic compounds (slow)
What is the historical and emerging understanding of soil organic carbon cycling?
Historical: the molecular structure determines the soil
Emerging: the soil conditions determine how long soil organic carbon can be stabilized in the soil
What is the effect of temperature on organic matter decomposition in soils?
The higher the temperature, the higher the decomposition rate, but only up to a certain point, then it decreases.
- decomposition occurs <0C
- optimal temperature at 25-30C
- increases up to 30C, then the decomposition rate starts to reduce until it stops
Enzymes denature at a certain point, they lose their structure and can’t perform. Microbes might start to die and lose efficiency.
What is the effect of moisture on organic matter decomposition in soils? What conditions refers to saturated, optimum, dry soil moisture content
In dry soil, decomposition rates are slow. The more water a soil has, the higher the decomposition rate.
Saturated soil: anoxic conditions (space is filled with water, there is no more oxygen)
Optimum water and air content: field capacity (sweet spot where there is sufficient air and oxygen and water for microorganisms in the pore space)
Dry soil: wilting point
Explain decomposition in anaerobic soils:
- O2 is depleted when soil pores are filled with water (saturated conditions)
- Without O2 aerobic organisms can not function (oxidation is not possible)
- Anaerobic or facultative organisms become dominant
- Products of anaerobic decomposition include organic acids, alcohols, and methane
- in dry soils, all of these products occur
- in wet soils (wetlands or prolonged rainfall), methane will dominate, it releases the most energy at that point
Why do anaerobic soils accumulate large amounts of partially decomposed organic matter?
- Decomposition takes place much more slowly
- Certain products of anaerobic metabolism are toxic to many microbes, acting as a preservative for organic matter
Discuss anaerobic decomposition and energy:
- Anaerobic decomposition releases little energy
- End products still contains much energy
- For this reason, acetate and methane, which are produced by anaerobic decomposition, can serve as fuel
Explain the relationship of the accumulation of soil organic matter through time:
At the start, production > decomposition. At a certain point, production = decomposition.
What is another other pathways for carbon loss?
Forest fires, which burn much of the forest floor. Slow build-up of organic matter in post-fire recovery of vegetation. This is important in boreal forests.
Explain losses of organic matter due to agriculture:
Agricultural systems have high plant productivity by much the organic matter removed in crops and frequent tillage and fertilizer application can speed up the rate of decomposition of organic matter.
Define biochar and black carbon. What is their relationship?
Black carbon is the byproduct of the chemical-thermal conversion of carbon-containing material.
Biochar refers to black carbon that is produced as a vehicle of carbon sequestration from biomass.
Therefore, biochar is black carbon, but not all black carbon is biochar.
What is pyrolysis and how is it produced?
Pyrolysis refers to the process of thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen.
Produced naturally through wildfires in grassland, forests etc., by slash and burn activities and deliberately by humans.
What are the effects of biochar on the environment and on crops yield/fertility?
- Generally raises soil pH
- May reduce nutrient losses through adsorption/immobilization
- High surface area and reactivity may lead to heavy metal (e.g. Pb, Cu, Ni, Cr, and Cd), phosphorus, and antibiotic sorption
- May reduce emission of N2O
- Increases water holding capacity of soils
- Not all soils will benefit 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.
Describe the global distribution of organic carbon in the upper 1m of the Earth’s soils. What drives these trends?
Lots of organic carbon in the tropics, northern forests (boreal forest). Very little in glacial or desert environments.
Climate is the leading factor for soil organic carbon. Water plays an important role.
- large primary production means more soil organic carbon is produced
- Desert: higher temperatures increase respiration and decomposition
- SOM increases with moisture (increased plant growth and leaching)
When considering microorganisms as an input of SOM, where do we find the highest amount of above ground biomass and below ground?
Above ground: tropical forests
Below grounds: temperate regions and semi arid grasslands regions (extensive root systems/root turnover and higher soil microbial biomass)
How has land use conversion affected the climate?
SOM is a source of atmospheric CO2. The additional CO2 in the atmosphere contributes to global warming due to the conversion of land such as deforestation.
What are the uses of stable and labile SOM?
Stabile: resistant and complex compounds (all grouped under humus), provide CEC (exchange cations in the soil solution) and water-holding capacity
- more structural plant litter like lignin
Labile: simple compounds, readily accessible food for organisms
- non structural components of plants: sugars and amino acids
Explain the carbon cycle process of SOM decomposition in soils.
Plant generate plant residue, which can lead to either simple components or resistant components.
The simple components are either directly mobilized by the microorganisms or mineralized by enzymatic activities. The end products are a bunch of nutrients which can be reused by plants or microorganisms.
Resistant components undergo slow decay and forms the humus. Humus is slowly mineralized because humus has complex molecules. Eventually these turn into simple end products are are reused by plants/microorganisms.
