Chapter 7 Terrestrial Decomposition

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Chapter 7Terrestrial Decomposition

• Part II Mechanisms
• Chapin, Matson, Mooney
• Principles of Terrestrial Ecosystem Ecology

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Atmospheric evidence of large carbon exchanges
by the biosphere
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Decomposition is physical and chemical breakdown
of dead organic matter

• Provides energy for microbial growth
• Releases nutrients for plant uptake
• Influences ecosystem carbon storage and therefore
  climate

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Decomposition consists of three processes

• 1. Leaching by water
• Transfers soluble materials
• 2. Fragmentation by soil animals
• Increases surface area for microbial attack
• 3. Chemical alteration
• Changes chemical composition of detritus

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Leaching

• Moves water-soluble compounds away from
  decomposing material
• Begins while leaves are still on plant
• Most important early in decomposition

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Fragmentation

• Fresh litter is protected from microbial attack
• Bark, epidermis or skin on exterior
• Plant cells protected by lignin in cell walls
• Carried out mainly by soil animals
• Increases surface area for microbial attack
• Important in aquatic and terrestrial ecosystems

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Chemical alteration

• Breaks down organic matter to CO2 and nutrients
• Forms complex recalcitrant compounds

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Soil is spatially heterogeneous

• Aboveground litter, organic mat, mineral soil
• Soil aggregates and macropores
• Rhizosphere and bulk soil

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Soil is chemically heterogeneous

• Fresh litter and old soil organic matter
• Different plant parts (leaves and wood)
• Cell walls and cell contents

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Who are the decomposers and why do the do it?
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Who are the decomposers and why do the do it?

• Decomposer organisms are subject to natural
  selection
• Decomposition is result of their feeding activity
  and population dynamics
• NOT a community service the carbon cycle
• They could care less about whether their activity
  promotes nutrient cycling and productivity of
  ecosystems

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Who are the major players in decomposition?
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Fungi

• Accounts for most decomposition in aerobic
  environments
• 60-90 of microbial biomass in forests
• About half of microbial biomass in grasslands
• Broad enzymatic capability
• Cell walls (lignin, cellulose, hemicellulose)
• Fungi are main lignin degraders
• Cell contents (proteins, sugars, lipids)

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Fungi (contd)

• Composed of long networks of hyphae
• Can transport metabolites through hyphae
• Surface litter (import nitrogen from soil)
• Wood degraders (import nitrogen from soil)
• Mycorrhizae (trade carbohydrates for nutrients)

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Bacteria

• Grow rapidly
• Specialize on labile substrates
• Some bacteria function anaerobically
• Dependent on substrates that diffuse to bacterium
  (not like fungi)
• Diffusion gradient caused by
• Production of soluble substrates (enzymes)
• Uptake of substrates by bacterium

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Bacteria (contd)

• Spatial specialists
• Rhizosphere, macropores, interior of aggregates
• form biofilms on particle surfaces
• Chemical specialists
• Different bacteria produce different enzymes
  (consortia)

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Bacteria (contd)

• Become inactive when substrate is exhausted
• 50 to 80 of soil bacteria inactive
• Activated by presence of substrate
• e.g., when root grows past

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Who are the major players in decomposition?
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Soil animals microfauna

• Protozoans (ciliates, amoebae)
• Aquatic, mobile
• Bacterial predators (phagocytosis)
• Rhizosphere specialists
• Nematodes (many trophic roles)
• Extremely abundant
• Often eat as much as aboveground grazers
• Mites (many trophic roles)

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Who are the major players in decomposition?
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Soil animals mesofauna

• Animals with greatest effect on decomposition
• Fragment litter
• Ingest litter particles and digest the microbial
  jam
• Collembolans are important mesofauna in northern
  soils

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Who are the major players in decomposition?
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Soil animals macrofauna

• Earthworms, termites, etc.
• Fragment litter or ingest soil
• Ecosystem engineers
• Mix soil, carry organic matter to depth
• Reduce compaction
• Create channels for water and roots

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Soil animals

• Account for only 5-10 of soil respiration
• Major impacts on decomposition are indirect
• Alter soil environment
• Graze bacteria and fungi
• Excrete nitrogen and phosphorus

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Litter mass declines almost exponentially with
time
k is the decomposition constant
-kt
L L e
t 0
litterfall
k
litterpool
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Decomposition is nearly exponential with
time Differs among substrates
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Phase 1 Leaching dominates Phase 2 High value
of k labile substrates broken down Phase 3 Low
value of k recalcitrant substrates
predominate Time scale depends on environment
(tropics vs. arctic)
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Controls over decomposition

• 1. Physical environment
• 2. Substrate quantity and quality
• 3. Properties of microbial community

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Controls over decomposition range from
long-to-short term Long-term State
factors Intermediate Interactive
controls Short-term Indirect and direct
physiological controls Direct controls
Environment and substrate quality
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Direct temperature effect on microbial
activity Temperature optimum is much higher
than ambient temperature Maintenance
respiration is increasing proportion of total
at high temperature High temperature not
necessarily optimal for microbes
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Direct temperature effects

• 1. Effects on microbial activity
• 2. Effect of temperature fluctuations
• e.g., freeze-thaw
• Kills microbes
• Spill their guts into the soil, where they become
  substrate for other decomposers
• Most decomposition at Toolik occurs in the
  non-summer season (Sarah Hobbie)

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Indirect temperature effects

• Effects on evaporation and soil moisture
• Effects on permafrost
• Changes in drainage
• Effects on quantity and quality of litter inputs

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Some of most important temperature effects are
indirect
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Moisture effects

• Decomposition has similar shape of moisture
  response as does NPP
• Declines at extremely low and high moisture
• Less sensitive to low moisture than is NPP (no
  litter accumulation in deserts)
• More sensitive to high moisture than is NPP (SOM
  accumulation in waterlogged soils)

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Other environmental effects

• pH
• bacteria predominate at high pH
• Low growth efficiency promotes breakdown
• Soil texture
• Protection of SOM by clays
• Aggregate structure (anaerobic microsites)

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Some chemical groups unavailable to enzymes when
organic matter binds to clay particles
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Other environmental effects

• Soil disturbance
• Reduces SOM protection by clays
• Breaks up soil aggregates

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Substrate quantity and quality are the major
controls over decomposition
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Substrate quality

• Susceptibility to decomposition
• May be THE predominant control over decomposition
• Climate exerts large effect on substrate quality
  through effects on vegetation

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Substrate quality depends on

• 1. Size of molecule
• 2. Types of chemical bonds
• 3. Regularity of structure
• 4. Toxicity
• 5. Nutrient concentrations

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Substrate quality depends on

• 1. Size of molecule
• Large molecules must be broken down outside of
  cells
• Limits metabolic control that microbes can exert
  over breakdown process
• Requires production of exoenzymes

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Substrate quality depends on

• 2. Types of chemical bonds
• Some bonds are easier to break than others
• e.g., peptide bonds compared to aromatic rings
• Most of litter nitrogen (80?) is in protein
• Most N in old SOM is in aromatic rings
• High N concentration in these two types of SOM
  means very different things to microbes

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Substrate quality depends on

• 3. Regularity of structure
• Lignin and humus have irregular structure
• 4. Toxicity
• Phenolics evolved to protect plants from
  herbivores and pathogens
• Also affect decomposers
• Importance of this effect is uncertain
• 5. Nutrient concentrations
• Nutrients are essential to support microbial
  growth

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Predictors of decomposition

• CN ratio
• Index of ratio of cytoplasm to cell walls
• Measure of nitrogen concentration
• Directly affects decomposition ONLY in presence
  of labile C
• LigninN ratio
• Integrated measure of N concentration and
  substrate size/complexity

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Plant species differ predictably in litter
quality High-resource-adapted leaves decompose
quickly due to higher concentrations of labile C
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Substrate quality of SOM

• Much of SOM is old and recalcitrant
• Consists of leftovers and microbial products
• Binds to clay minerals
• Bulk soil is a nutritional desert

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Rhizosphere is major zone of decomposition in
mineral soil High inputs of labile C prime
decomposition Microbes break down SOM for
nitrogen
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Long-term storage of SOM Humus
formation Formation of SOM that doesnt
decompose easily Critical determinant of soil
properties
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Controls over stand-level decomposition are
similar to Controls of GPP and NPP
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Major controls over decomposition

• Quantity of litter input
• Quality of litter input
• Environmental conditions that favor biological
  activity

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Soil respiration correlates closely with NPP
because Some (75) is decomposition, which
depends on litter quantity and quality Some
(25) is root respiration which correlates
with NPP
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Summary

• Decomposition is the major avenue of carbon loss
  from ecosystems
• Determined primarily by same factors that
  regulate NPP
• Sensitive to global change
• Has potentially large feedbacks to climate

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