Decomposition

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.).

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. 

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 (C:N ratio)– Etc.– season, tillage

Litter Bag removed from Soil

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 (C:N ratio)– Etc.– season, tillage

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

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 (C:N ratio)– Etc.– season, tillage

• Season: spring = fresh green material; fall = tough residues

• Tillage: aerates soil

C:N ratio

• Slower decomposition if C:N ratio is high.• Examples of C:N ratios…..

C:N Ratios of Selected Materials

Importance of C:N ratio

• With residues of high C:N, there is much 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).

Importance of C:N Ratio

Organic N

Inorganic N

Mineralization

Immobilization

Bacteria take scarce N for themselves !

Bacteria free up N

Adding organic N to soil

Organic N materials decompose, release nitrate

Adding organic N to soil

Organic N materials decompose, release nitrate

Nitrate used in decomposition process by bacteria, etc.

Adding organic N to soilLow C:N ratio material

Decomposition and release of nitrate is fast, levels recover quickly

Adding organic N to soilHigh 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

Nitrate release difficult, tied up for a long time

Risky Mix of High and Low C:N

• High C:N residue +

• Inorganic N fertilizer =

• N from fertilizer immobilized by bacteria

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.

Composting

• Accelerates decomposition process

• Reduces C:N ratio

• High temperatures – 60-80oC (140-175oF)

• Thermophilic bacteria

• Many recipes available

Sequence of Events in Decomposition

• Applies to OM of plant origin• OM of animal origin has low C:N, easily

decomposed by animals, etc.

soil animals bacteria, fungi soil chemistryOrganic detritus humus mineralizationmaterial (small particles)(litter)

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

References

• Text, pp. 68-73.• Brady and Weil, 1996. The Nature and

Properties of Soils. Prentice Hall, Upper Saddle River, NJ.

• Odum, 1983.