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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.