1. Decomposition

2. Decomposition Breakdown of organic matter, recycles nutrients.
Important for recycling of C and energy, as well as all nutrient elements (N, P, K, etc.).
Determines productivity and health of plants.

3. Biotic Decomposition
Organisms most directly responsible for decomposition are bacteria and fungi. 
Biotic processes:
Aerobic respiration
OM + O2  CO2, organic products.
Anaerobic respiration
Use of acceptors other than oxygen.
Facultative or obligate anaerobes
OM + CO2  CH4 + organic products.

4. Sequence – Biotic Decomposition
Humus
Inorganic compounds
Living organisms
Dead organisms
Litter OM
Detritus

5. Biotic decomposition
Many organisms are very beneficial in ecosystems, participate in biotic decomposition.

6. Sequence – Biotic Decomposition
Living organisms
Dead organisms
Litter OM
Detritus
Humus
Inorganic compounds
Soil Animals
Bacteria, fungi
Soil chemistry, mineralization

7. 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.

8. Measuring Decomposition
Litter bag method.

9. Decomposition Rates Decomposition rates vary depending on:
Climate and temperature
Soil organisms present
Aeration of the soil
Composition of the material (C:N ratio)
Etc.– season, tillage

10. Decomposition Rates Decomposition rates vary depending on:
Climate and temperature
Soil organisms present
Aeration of the soil
Composition of the material (C:N ratio)
Etc.– season, tillage
Temperature affects decomposition rates (heat units).
Less Organic Matter in warm climates.

11. Decomposition Rates Decomposition rates vary depending on:
Climate and temperature
Soil organisms present
Aeration of the soil
Composition of the material (C:N ratio)
Etc.– season, tillage

12. Decomposition Rates Decomposition rates vary depending on:
Climate and temperature
Soil organisms present
Aeration of the soil
Composition of the material (C:N ratio)
Etc.– season, tillage
Season: spring = fresh green material; fall = tough residues.
Tillage: aerates soil.

13. Decomposition Rates Decomposition rates vary depending on:
Climate and temperature
Soil organisms present
Aeration of the soil
Composition of the material (C:N ratio)
Etc.– season, tillage

14. C:N ratio
Most important factor influencing the rate of decomposition of OM.
Ratio of amount of carbon to the amount of nitrogen in a substance.
Slower decomposition if C:N ratio is high.
Examples of C:N ratios…..

15. C:N Ratios of Selected Materials
20:1
for crops

16. Importance of C:N ratio
With residues with a C:N ratio that is high, there is competition for the limited N available.
If C:N 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).

17. Adding organic N to soil
Organic N materials decompose, release nitrate.
Uptake, leaching, denitrification

18. Adding organic N to soil
Organic N materials decompose, release nitrate.
Nitrate used in decomposition process by bacteria, etc.

19. Adding organic N to soil Low C:N ratio material
Decomposition and release of nitrate is fast, levels recover quickly.

20. Adding organic N to soil High C:N ratio material
Nitrate release difficult, tied up for a long time.
N rob = immobility of N in presence of residues with high C:N ratios.

21. Importance of C:N Ratio
Bacteria free up N
Organic N
Inorganic N
Mineralization
Immobilization
Bacteria take scarce N for themselves ! Nrob

22. Risky Mix of High and Low C:N
High C:N residue + Inorganic N fertilizer =
N from fertilizer immobilized by bacteria.

23. Importance of C:N ratio
As C:N 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 C:N ratio of their food is low:
E.g., fungi 10:1, decomposed animals 6:1, nematodes 10:1.

24. Composting Accelerates decomposition process. Reduces C:N ratio.
High temperatures – 60-80oC ( oF).
Thermophilic bacteria.
Many recipes available.

25. Practical Application
Farmer incorporates 2560 kg of OM to soil. 55% Carbon, C:N ratio = 20:1.
Will there be any available for crop or all immobilized?
2560 X 0.55 = 1408 kg C / 20 = 70 kg N
Microbes use 75% of C and incorporate 25% in new tissue. C:N ratio of microbe is 8:1, so they require 1 kg N to incorporate 8 kg C in new tissue.
1408 / 0.25 = 352 kg C used in new tissue.
352 / 8 = 44 kg N used in new tissue.
70 – 44 = 26 kg N leftover for uptake by plants.

26. References
Text, pp
Brady and Weil, The Nature and Properties of Soils. Prentice Hall, Upper Saddle River, NJ.
Odum, 1983.