lecture 5

Question 1

What is the importance of soil organic matter in soils?

Answer 1

1. Helps develop structural strength through bonding
2. Improves porosity and pore size distribution, which increases infiltration rate, permeability and water availability
3. Releases nutrients through decomposition
4. Possesses cation exchange capacity to supply nutrients to plants and store cations added as fertilizer, as well as buffering capacity

Question 2

What does soil organic matter content depend on? What’s missing?

Answer 2

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.

Question 3

What are the inputs of organic matter to soils?

Answer 3

• Above-ground plant inputs: leaves, needles, wood
• Below-ground plant inputs: roots (coarse and fine)
• Microfauna: bacteria and fungi
• Macrofaune: anthropods and earthworms

Question 4

What are the outputs of soil organic carbon?

Answer 4

• Decomposition (root respiration and microbial respiration)
• Leaching
• Erosion
• Crop harvest
• Deforestation
• Forest fires

Question 5

What happens in well-aerated soil?

Answer 5

In well-aerated soil, all of the organic compounds found in plant residues are subject to oxidation.

Decomposition is an oxidation.

Question 6

What are the controls on the rate of organic matter decomposition in soils?

Answer 6

• Composition of plant tissue and organic matter
• Temperature
• Moisture
• Nutrient content of soil
• Macro and micro organisms in soil

Question 7

What are the elements that go trough the fastest to lowest decomposition?

Answer 7

1. Sugars, starches, simple proteins (rapid, rich in nitrogen, phosphorus, all nutrients required for microbial growth)
2. Crude proteins
3. Hemicellulose
4. Fats and waxes
5. Lignins and phenolic compounds (slow)

Question 8

What is the historical and emerging understanding of soil organic carbon cycling?

Answer 8

Historical: the molecular structure determines the soil

Emerging: the soil conditions determine how long soil organic carbon can be stabilized in the soil

Question 9

What is the effect of temperature on organic matter decomposition in soils?

Answer 9

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.

Question 10

What is the effect of moisture on organic matter decomposition in soils? What conditions refers to saturated, optimum, dry soil moisture content

Answer 10

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

Question 11

Explain decomposition in anaerobic soils:

Answer 11

1. O2 is depleted when soil pores are filled with water (saturated conditions)
2. Without O2 aerobic organisms can not function (oxidation is not possible)
3. Anaerobic or facultative organisms become dominant
4. 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

Question 12

Why do anaerobic soils accumulate large amounts of partially decomposed organic matter?

Answer 12

1. Decomposition takes place much more slowly
2. Certain products of anaerobic metabolism are toxic to many microbes, acting as a preservative for organic matter

Question 13

Discuss anaerobic decomposition and energy:

Answer 13

• 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

Question 14

Explain the relationship of the accumulation of soil organic matter through time:

Answer 14

At the start, production > decomposition. At a certain point, production = decomposition.

Question 15

What is another other pathways for carbon loss?

Answer 15

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.

Question 16

Explain losses of organic matter due to agriculture:

Answer 16

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.

Question 17

Define biochar and black carbon. What is their relationship?

Answer 17

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.

Question 18

What is pyrolysis and how is it produced?

Answer 18

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.

Question 19

What are the effects of biochar on the environment and on crops yield/fertility?

Answer 19

• 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.

Question 20

Describe the global distribution of organic carbon in the upper 1m of the Earth’s soils. What drives these trends?

Answer 20

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)

Question 21

When considering microorganisms as an input of SOM, where do we find the highest amount of above ground biomass and below ground?

Answer 21

Above ground: tropical forests

Below grounds: temperate regions and semi arid grasslands regions (extensive root systems/root turnover and higher soil microbial biomass)

Question 22

How has land use conversion affected the climate?

Answer 22

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.

Question 23

What are the uses of stable and labile SOM?

Answer 23

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

Question 24

Explain the carbon cycle process of SOM decomposition in soils.

Answer 24

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.

Question 25

Would leaves from a forest or a swamp decompose slower?

Answer 25

The leaves from swamps decompose slower because swamps are anaerobic, they are water-saturated so there are no molecular oxygens that can be used for the oxidation process of decomposition

Beech leaves in particular decompose slower because they have a thicker wax on their leaves, making the rate of decomposition slower.

Question 26

What is the significance of the exponential decay constant? What type of biomass has the highest k constant?

Answer 26

The higher the constant, the slower the decomposition.

Tropical forest leaves have the highest constant aka decompose the slowest because they are very waxy (bc they want the rain to fall off their leaves so that no parasites or fungi can develop)

Question 27

Explain how the nutrient content of the soil influences SOM decomposition.

Answer 27

Microbial communities compete for nitrogen in the soil. If the organic matter has a high C:N ratio, microbes may struggle to obtain enough nitrogen.

Higher C:N ratios will decompose slower in the soil, leaving more mass overtime after placing it on a soil.
- examples: decidious leaves, evergreen needles, roots, wood

If there is a high availability of nitrogen, there will be a rapid decomposition process.
- examples: bacteria, fungi, insects/mammals

Question 28

What are K and R strategists? What is the priming effect?What is there relationship?

Answer 28

K strategists: focus on using soil humus, not easily decomposable organic matter

R strategists: kick in when there are easily available compounds

Priming effect: losing part of the humus to the R strategists

R strategists start to produce lots of CO2 because of the availability of compounds, which leads to the priming effect and a decline in K strategists because they are outcompeted. Overtime, K strategists become more competitive once the easily available compounds are no longer available. This produces a new soil humus level, which is higher than the old soil humus level.

Question 29

Summarize the process of SOM.

Answer 29

1. Mechanical shredding of litter by soil fauna
2. Carbon compounds are enzymatically oxidized to produce carbon dioxide, water and energy
3. Nutrients such as nitrogen, phosphorus, and sulfur are released and/or immobilized
4. New compounds are synthesized by microbes as cellular constituents or as breakdown products
5. Some of the original plant compounds, their breakdown products and microbial compounds becomes physically or chemically protected from further microbial decay via interactions with the soil minerals