Transcript Decomposition

Decomposition
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.
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.
Sequence – Biotic Decomposition
Living
organisms
Dead
organisms
Litter
OM
Detritus
Humus
Inorganic
compounds
Biotic decomposition
Many organisms are very beneficial in
ecosystems, participate in biotic
decomposition.
Sequence – Biotic Decomposition
Living
organisms
Dead
organisms
Litter
OM
Detritus
Soil Animals
Humus
Bacteria,
fungi
Inorganic
compounds
Soil chemistry,
mineralization
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.
Measuring Decomposition
Litter bag method.
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
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.
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
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.
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
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…..
C:N Ratios of Selected Materials
20:1
for
crops
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).
Adding organic N to soil
Organic N materials
decompose, release
nitrate.
Uptake, leaching,
denitrification
Adding organic N to soil
Nitrate used in
decomposition
process by
bacteria, etc.
Organic N materials
decompose, release
nitrate.
Adding organic N to soil
Low C:N ratio material
Decomposition and
release of nitrate is
fast, levels recover
quickly.
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.
Importance of C:N Ratio
Organic N
Bacteria
free up N
Inorganic N
Mineralization
Immobilization
Bacteria take
scarce N for
themselves !
Nrob
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.
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
1408 / 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.
References
Text, pp. 68-73.
Brady and Weil, 1996. The Nature and
Properties of Soils. Prentice Hall, Upper
Saddle River, NJ.
Odum, 1983.