Theme 5

Question 1

What are the main components of soil organic matter (SOM)?

Answer 1

Biomass, detritus, soluble SOM, unprotected SOM, protected SOM, and colloidal SOM.

Question 2

What is biomass in SOM?

Answer 2

Living soil organisms (e.g., microbes, fungi, roots) that actively contribute to nutrient cycling.

Question 3

What is detritus in SOM?

Answer 3

Dead plant and animal residues that provide energy for decomposers.

Question 4

What is soluble SOM?

Answer 4

Dead plant and animal residues that provide energy for decomposers.

Question 5

What is soluble SOM?

Answer 5

Readily decomposable compounds (e.g., sugars, amino acids) that cycle quickly in the soil.

Question 6

How does unprotected SOM differ from protected SOM?

Answer 6

Unprotected SOM decomposes quickly, while protected SOM is stabilized through physical, chemical, or biological mechanisms.

Question 7

What is colloidal SOM?

Answer 7

Fine organic particles dispersed in soil solution, contributing to nutrient transport.

Question 8

How does detritus protect soil surfaces?

Answer 8

By controlling erosion, regulating temperature, retaining water, and supporting microbial activity.

Question 9

What are the three mechanisms of SOM protection?

Answer 9

Physical protection (aggregation), chemical protection (mineral binding), and biochemical protection (complex compounds).

Question 10

How does clay content affect SOM protection?

Answer 10

High clay content stabilizes SOM, reducing decomposition and enhancing carbon sequestration.

Question 11

What are the fundamental processes of organic matter decomposition?

Answer 11

Microbial activity, oxidation, and energy release.

Question 12

What role do heterotrophic organisms play in SOM turnover?

Answer 12

They break down organic residues, convert them into microbial biomass and CO₂, and drive decomposition.

Question 13

What factors influence decomposition rates?

Answer 13

C:N ratio, biochemical composition, moisture, aeration, and pH.

Question 14

How does C:N ratio affect decomposition?

Answer 14

High C:N (e.g., wood) slows decomposition, while low C:N (e.g., manure) speeds it up.

Question 15

What organic compounds decompose at different rates?

Answer 15

• Fast: Simple sugars, proteins

Moderate: Hemicellulose, cellulose
Slow: Lignin, polyphenols

Question 16

How does lignin affect decomposition?

Answer 16

Lignin slows decomposition by forming complex structures that resist microbial breakdown.

Question 17

What is humus, and why is it important?

Answer 17

Humus is a stable organic compound that enhances carbon sequestration, soil fertility, and structure.

Question 18

What are the key principles of composting?

Answer 18

Thermophilic & mesophilic phases, optimal moisture (50–60%), aeration, and a C:N ratio of ~25–30:1.

Question 19

What are the physical benefits of SOM?

Answer 19

Improves soil structure, water retention, and temperature regulation.

Question 20

What are the chemical benefits of SOM?

Answer 20

Enhances cation exchange capacity (CEC), buffers pH, and supplies nutrients.

Question 21

What are the biological benefits of SOM?

Answer 21

Promotes microbial activity and supports biodiversity.

Question 22

How does SOM contribute to soil fertility?

Answer 22

It enhances CEC, buffers pH, and facilitates nutrient cycling.

Question 23

How does SOM improve soil stability and resilience?

Answer 23

Increases erosion resistance, prevents compaction, and enhances aeration.

Question 24

How does SOM support carbon sequestration?

Answer 24

Stores carbon long-term, reducing atmospheric CO₂ and improving soil productivity.

Question 25

What environmental factors affect SOM content?

Answer 25

Temperature, moisture, and soil texture.

Question 26

How does landscape position influence SOM accumulation?

Answer 26

Steep slopes lose SOM due to erosion, while gentle slopes accumulate more.

Question 27

How does soil texture influence SOM stability?

Answer 27

Clay-rich soils protect SOM; sandy soils have poor retention.

Question 28

How does land management affect SOM?

Answer 28

Tillage depletes SOM; no-till farming and cover crops enhance it.

Question 29

What are global trends in SOM distribution?

Answer 29

Tropical regions: High decomposition, low SOM storage Temperate regions: Moderate accumulation Cold regions: High SOM storage in peatlands

Question 30

What are the two main forms of carbon in soil?

Answer 30

Soil Organic Carbon (SOC) and Soil Inorganic Carbon (SIC).

Question 31

How is SOC accumulated and stabilized?

Answer 31

Through plant residues, microbial activity, and physical, chemical, or biochemical stabilization.

Question 32

What processes contribute to SIC accumulation?

Answer 32

Carbonate precipitation in alkaline soils.

Question 33

What is the role of soil in the global carbon cycle?

Answer 33

Stores more carbon than the atmosphere and vegetation, playing a key role in carbon sequestration.

Question 34

How does climate change impact soil carbon stocks?

Answer 34

Warming accelerates SOC decomposition; permafrost thawing releases stored carbon.

Question 35

How does soil contribute to greenhouse gas emissions?

Answer 35

CH₄: Produced under anaerobic conditions CO₂: Released from microbial decomposition

Question 36

How does land-use change affect soil carbon dynamics?

Answer 36

Deforestation and tillage reduce SOC, while afforestation and conservation practices restore it.