Pseudo clicker Q:

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Pseudo clicker Q The amount of fixed energy
available to heterotrophs (on a sustainable
basis) is called_____________ The amount of
this energy that feeds the herbivore food web
averages about _________ of plant productivity
in forests and . The remaining energy feeds
the ________________________
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Plant allocation strategies control the quality
of dead organic matter.
(Modified from Aber and Melillo, 1991)
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Plants dont throw away good stuff (except
maybe to assist in reproduction) As the leaves,
stems and roots of plants senesce, the plant
uses energy to disassemble proteins and resorb
materials that can be transported in
phloem tissue. Structural material (cellulose,
lignin, and materials embedded in these
materials) is NOT resorbed. Its not a perfect
process. For example, senescent foliage usually
has about half of the nitrogen content of
live foliage.
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Reallocation, translocation
Plants burn energy to internally recycle
nutrients (esp N, P)
(Modified from Aber and Melillo, 1991)
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Plant litter (senesced, dead plant material)
tends to be nutrient-poor and made up of
complex (structural and defense chemical)
molecules. In contrast, herbivore-mediated
litter (largely in the form of fecal material)
often has a chemistry that more closely matches
live plant material. It tends to be a higher
quality resource for decomposers than is plant
litter. From this perspective.herbivores make
high quality litter. However...
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.
Plants also allocate to materials to protect
their investments secondary plant
chemicals.
(Modified from Aber and Melillo, 1991)
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This material is also an evolutionary consequence
of plants attempt to discourage pathogens and
herbivory (from Aber and Melillo 1991)
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Large molecule anti-herbivore defenses have a
negative effect on the speed at which dead
organic matter is consumed. Lignins are not
assimilated by any herbivores and very few
decomposer organisms because of its size
and structural complexity. However, a few
microbes bacteria and fungido produce
EXOENZYMES that are at least partially effective
at breaking this stuff down to manageable
pieces. This slow process allows for the
accumulation of soil organic matter.
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Decomposition f (substrate quality)

• Litter Quality depends on
• chemical bonds
• size and 3-dimensional complexity
• nutrient content

Accessible to breakdown by exo- or endo- enzymes.
For energy gain Energy gain must offset high
cost of enzyme production. e.g. Bonds in simple
sugars are energy-rich yummy.
Decomposer organisms need nutrients for growth.
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Procedure for measuring decomposition and
mineralization with litterbags. Initial mass
100 Collected mass of original Percent
remaining Collected amount/Initial Amount
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Under a constant climate, a single substrate
quality, And a fixed biotic community Mass loss
(decomposition) is a negative exponential
function. (Where have we seen that function
describe an ecological process before?)
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Decomposition is nearly exponential with
time Differs among substrates
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How important Are these?
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Who are the decomposers and why do they do it?
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Who are the decomposers and why do they 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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Life in a litterbag Mass loss in
a litterbag Mass loss
Respiration
Microflora
Litter
Microbivores and Detritivores
Fragmentation and leaching
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Who are the major players in decomposition? Fig.
7.1
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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? Fig.
7.1
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Soil animals microfauna

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

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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, as are the mites

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How many per square meter of soil?
____________________________________
Individuals
Species ____________________________________ Nema
todes 3,000,000
200? Arthropods 200,000
100? ____________________________________
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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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LifeLilife in a litterbag Mass loss in a
litterbag Mass loss
Respiration
Microflora
Litter
Microbivores and Detritivores
Fragmentation and leaching
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Microbe Respiration
0
Density of microbe predators (invertebrates)