5-2 Decomposition

Question 1

What is the composition of soil?

Answer 1

Inorganic materials, organic materials, air, water, organisms (plants, bacteria, fungi, protists, animals)

Question 2

What is the size of the global soil OM pool?

Answer 2

• 2500 Gigtions of carbon
• Soil Organic Carbon is GT (62%)
• Remaining is carbonates (CaCO3)
• 1 gigation = 1 empire state building

Question 3

Examples of strategies for maintenance and increase of soil organic matter (SOM):

Answer 3

Minimize losses: erosion, limit cultivation, limit overgrazing

Increases storage: amend soil with OM, practice conservation tillage, maintain plant cover, plant deep-rooted perennials, high plant diversity, high soil microbial diversity

Question 4

Mineralization

Answer 4

Organic nutrients (N or P) will turn into inorganic nutrients

Question 5

Immobilization

Answer 5

turns inorganic nutrients to organic nutrients

Question 6

Benefits of OM:

Answer 6

• carbon sink
• storage of nutrients (especially in and on aggerates)

Question 7

What are the biological conditions associated with the breakdown of OM?

Answer 7

• Detrivitores: larger organisms break dead tissues into small pieces; include inscets, isopods, worms, nematodes etc
• Decomposers: bacteria, protists & fungi that consume the small bits of organic matter, some carbon released as CO2 (respired)

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Question 8

What are the physical factors that contribute to the breakdown of OM?

Answer 8

• temperature
• moisture (O2 availability, lack of slowed decomposition)
• O2
• tillage (starts decomposition)
• lignin content

Question 9

How does CN ratio within OM contribute to its decomposition rate?

Answer 9

high CN litter decomposes slowly, and low CN litter decomposes rapidly

Question 10

How does the CN ratio of an amendment (and composition with bacteria) alter plant nutrient availability?

Answer 10

• adding OM to soil can alter the physical and nutrient environment (porosity (number of air spaces) and nutrient availability)
• ex: mulching, cover crops, and residue retention

Question 11

How does the microbial loop or plant exudates increase OM mineralization?

Answer 11

• if OM is high C:N ratio, bacteria (which grows faster) will steal nitrogen from plants
• High CN ratio 5:1: wood chips 500:1 (immobilization)
• if OM has a Low C:N ratio bacteria will decompose other (more resistant) organic matter
• Low CN ratio 2:1: raw manure 2:1 (mineralization)

Fungi have a higher CN ratio (~20:1)

Question 12

CN ratio for the baseline:

Answer 12

• C:N > 30:1 immobilization likely (N bound by microbes
• C:N < 20:1 immobilization unlikely (N available to plants)
• C:N between 20:1 and 30:1 generally balanced

Question 13

Compare and contrast microbial loop, litter homefield, and feedback:

Answer 13

• Microbial loop: plants exclude carbon from roots, 50% of carbon fixed, rhizosphere ‘hot spot’ for microbes, simple sugar w/ decomposition, waste of microbes contains nitrogen, plants regulate
• Homefield advantage: plants ary in litter quality (CN ratio), plants litter can also contain chemicals (nicotine), complex communities
• feedback: plants interact with microbes, can help maintain plant species (mycorrhizal fungi, rhizobia), can cause plant turnover (diseases, parasitic nematodes)

Question 14

Mycorrhizae - a symbiotic relationship

Answer 14

• various forms (ecto- endo- vesiular-arbuscular)
• can increase root surface area >700%
• plants provide sugars (carbon)
• fungi provides nutrients and water (P especially)