21: Biogeochemistry

Question 1

What % of soil organic matter is carbon?

Answer 1

58%

Question 2

Carbon storage of soils is the result of inputs and outputs, what are they?

Answer 2

Inputs include plant growth, animal materials, and microbial necromass (dead microbes). Outputs are due to decomposition and erosion.

Question 3

What is meant by molecular complexity of organic compounds?

Answer 3

Size, weight, bonds, aromaticity

Question 4

What is meant by recalcitrance of organic compounds?

Answer 4

Recalcitrance indicates the strength of the bonds, robustness of the molecule, and resistance to degradation

Question 5

Decomposition

Answer 5

The process that transforms organic materials from identifiable plant, animal, and microbial residues into CO2, inorganic nutrients, and humus.

Decomposition is done indirectly through larger soil organisms and directly by soil microorganisms.

Question 6

Biochemical decomposition

Answer 6

First starts with depolymerization, where microbial cells release enzymes to the environment that start to break down large molecules into monomeric units. Then the cells take those monomeric units in to use

Question 7

Factors leading to faster and slower decomposition

Answer 7

Smaller particle size results in faster decomposition
Recalcitrance leads to slower decomposition

Question 8

Nitrogen immobilization

Answer 8

Capture of available inorganic N by soil microbes when organic material with a high C:N ratio (>30) is added to the soil

Question 9

Specific C:N ratio of soil microorganisms

Answer 9

Bacteria: 5-8
Fungi: 9-22

Can only break down 5-8 parts of carbon, the rest will be left and decomposed

Question 10

C stabilization

Answer 10

Chemical stabilization
- Flocculation

Physical stabilization
- Organic molecules become concealed or covered by mineral particles in soil aggregates, protecting them from being decomposed by microbes

Question 11

Factors regulating decomposition

Answer 11

Biotic, physiochemical, soil properties, climate, C stabilization, and management

Question 12

Biotic factor in regulating decomposition

Answer 12

Particle size
- Smaller size = faster decomposition

Question 13

Physiochemical factors in regulating decomposition

Answer 13

More complex molecules decompose slower because of recalcitrance
Soil microorganisms have a specific C:N ratio in their own biomass
- Bacteria: 5-8
- Fungi: 9-22

They will only take up C and N in these ratios depending on the ratio of C:N in their own biomass. If there is a high C:N ratio, that means there will be slow decomposition and N immobilization

Question 14

Soil properties role in regulating decomposition

Optimal pH for bacteria and fungi
How does oxygen availability affect decomposition, can bacteria and fungi both decompose in anaerobic/aerobic conditions?
Other considerations

Answer 14

pH:
- Bacteria: 6.5-8 pH
- Fungi: 5.5-6.5

Oxygen availability:
- Decomposition slower in absence of oxygen
- Bacteria can decompose things in anaerobic conditions but fungi need aerobic conditions

Other considerations
- Decomposition is optimal in moist but not water saturated conditions
- Microorganisms also need N, P, and other nutrients to carry out decomposition

Question 15

Climate factor in regulating decomposition

What temperatures do bacteria grow at? What’s their optimal temperature?

Answer 15

Bacteria grow at 4-60°C, optimal temp is 20°C (68°F). They also need adequate amounts of moisture

Question 16

C stabilization (C sequestration) factor in regulating decomposition

Why does stabilization lead to carbon sequestration?
What is physical and chemical stabilization?

Answer 16

The decomposition and production of microbial necromass leads to stabilization. The stabilized carbon is able to be further decomposed and thus leads to carbon sequestration

Chemical stabilization
- Flocculation: OM forms a “flock” with minerals, decreasing availability to microbes

Physical stabilization
- Organic molecules become concealed or covered by mineral particles in soil aggregates, protecting them from being decomposed by microbes

Question 17

Management factor in regulating decomposition

Answer 17

C storage in soils is the result of the balance between inputs and outputs