Decomposition

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Decomposition
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Decomposition

• Many organisms are very beneficial in ecosystems
  as decomposers.
• Breakdown of organic matter recycles nutrients
• Decomposition important for recycling of C and
  energy, as well as all nutrient elements (N, P,
  K, etc.).

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Types of Decomposition

• Abiotic processes - fire, etc. 
• OM O2 ? CO2, etc.
•  Biotic processes
• Aerobic respiration
• OM O2 ? CO2, etc.
• Anaerobic respiration
• OM ? CH4, etc.
•  Organisms most directly responsible for
  decomposition are bacteria and fungi. 

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Decomposition Rates

• Decomposition rates vary (measured in litter
  bags), depending on
• Climate and temperature
• Soil organisms present
• Aeration of the soil
• Composition of the material (CN ratio)
• Etc. season, tillage

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Litter Bag removed from Soil
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Decomposition Rates

• Decomposition rates vary (measured in litter
  bags), depending on
• Climate and temperature
• Soil organisms present
• Aeration of the soil
• Composition of the material (CN ratio)
• Etc. season, tillage

Temperature affects decomposition rates (heat
units) Less Organic Matter in warm climates
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Decomposition Rates

• Decomposition rates vary (measured in litter
  bags), depending on
• Climate and temperature
• Soil organisms present
• Aeration of the soil
• Composition of the material (CN ratio)
• Etc. season, tillage
• Season spring fresh green material fall
  tough residues
• Tillage aerates soil

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CN ratio

• Slower decomposition if CN ratio is high.
• Examples of CN ratios..

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CN Ratios of Selected Materials
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Importance of CN ratio

• With residues of high CN, there is much
  competition for the limited N available.
• If CN ratio is high
• only some organisms can decompose (some types of
  bacteria and fungi, protozoan symbionts of
  termites).
• they use up N quickly, so N becomes tied up and
  unavailable (immobilized).

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Importance of CN Ratio
Bacteria free up N
Organic N
Inorganic N
Mineralization
Immobilization
Bacteria take scarce N for themselves !
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Adding organic N to soil
Organic N materials decompose, release nitrate
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Adding organic N to soil
Organic N materials decompose, release nitrate
Nitrate used in decomposition process by
bacteria, etc.
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Adding organic N to soilLow CN ratio material
Decomposition and release of nitrate is fast,
levels recover quickly
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Adding organic N to soilHigh CN ratio material
Nitrate release difficult, tied up for a long time
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N rob immobility of N in presence of residues
with high CN ratios
Nitrate release difficult, tied up for a long time
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Risky Mix of High and Low CN

• High CN residue
• Inorganic N fertilizer
• N from fertilizer immobilized by bacteria

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Importance of CN ratio

• As CN ratio goes lower (as it does during
  decomposition), more organisms (especially fungi,
  different bacteria types, even some insects) can
  join in, so decomposition proceeds much faster.
• Most insects and other animals join later in the
  process, as the CN ratio of their food is low
• e.g., fungi 101, decomposed animals 61,
  nematodes 101.

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Composting

• Accelerates decomposition process
• Reduces CN ratio
• High temperatures 60-80oC (140-175oF)
• Thermophilic bacteria
• Many recipes available

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Sequence of Events in Decomposition
soil animals bacteria, fungi soil
chemistry Organic ? ? ? detritus ? ?
? humus ? ? ? mineralization material
(small particles) (litter)

• Applies to OM of plant origin
• OM of animal origin has low CN, easily
  decomposed by animals, etc.

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Sequence of Events in Decomposition

• Animals speed decomposition
• break litter into small pieces.
• add products (excretion) to stimulate microbial
  growth.
• feeding on fungi and bacteria often increases
  growth rate of these decomposers, since new
  surface area is exposed to decomposition.
• Predators regulate densities of animals that
  consume bacteria and fungi.
• Move decomposers (bacteria, fungi) to new
  locations

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References

• Text, pp. 68-73.
• Brady and Weil, 1996. The Nature and Properties
  of Soils. Prentice Hall, Upper Saddle River, NJ.
• Odum, 1983.