Detritus Food Chains and Biogeochemical Cycles Lecture 8 Chapters 21 and 22

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Detritus Food Chains and Biogeochemical
CyclesLecture 8Chapters 21 and 22
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• 1. Much recycling of materials occurs within
  organisms
• Example water/nutrients withdrawn from senescent
  leaf tissues of plants ? roots
• Dead matter broken down via detritral food chain
• Decomposition multistep process leading to
  mineralization
• mineralization conversion of organic nutrients
  to mineral form
• Terms
• Fixation incorporation to organic molecule
• Mineralization reduction of organic molecule
  returning component elements to inorganic
  associations

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• Life depends on recycling chemical elements
• Nutrient circuits in ecosystems involve biotic
  and abiotic components and are often called
  biogeochemical cycles
• Focus on
• Each chemicals biological importance
• Forms in which each chemical is available or used
  by organisms
• Major reservoirs for each chemical
• Key processes driving movement of each chemical
  through its cycle

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Fixation
Mineralization
Exchange Pool
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• Two food chains
• Grazing food chain
• Herbivore ? carnivore
• Detritus food chain
• Dead matter and waste from grazing food chain and
  primary production
• Provides input to grazing food chain

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• Detrivore food chain
• heterotrophs feed on dead material
• Provide prey in herbivore foodchain
• Fragmentation
• Microfauna and flora lt100um
• Protozoans and nematodes
• Mesofauna 100um? 2mm
• Mites, potworms, springtails
• Macrofauna
• Millipedes, earthworms, snails, amphipods
  isoods
• Decomposition
• Bacteria and fungi produce extracellular
  enzymes

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• Fungi belong to a separate kingdom
• several groups
• produce long, thread-like strands (hyphae)
• reproductive structures may be large and visible

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• Bacteria two distinct kingdoms
• Single celled
• Microscopic
• Various shapes
• Many may not be easily cultured
• May develop populations quickly

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• Study of Decomposition Litterbag Studies
• Weighed sample in mesh bag placed in soil
• Withdrawn after time to determine remaining
  dry-weight
• Dry weight estimate distorted by biomass of
  decomposer
• Gives estimate of decomposition impacted by
• Species
• conditions

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Other factors which may impact rate of
decomposition?
Decomposition of red maple leaves more rapid in
warmer, more humid climatesdde
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• Decomposition of different species and materials
  vary
• Simple sugars ? bacteria and fungi
• polymers (as cellulose) ? mainly fungi, some
  bacteria
• Lignins ? only certain fungi

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Lignin the stuff of wood, slow to degrade and
degredation rate levels off as it remains
Cellulose goes second, degraded by bacteria and
fungi
Proteins, solub. Carbohydrates easily degraded
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Fig. 55-15
Ecosystem type
EXPERIMENT
Arctic
Subarctic
Boreal
Temperate
A
Grassland
Mountain
G
M
D
B,C
P
T
H,I
E,F
S
O
L
N
U
J
K
R
Q
RESULTS
80
70
U
60
R
O
Q
K
50
T
Percent of mass lost
J
P
40
S
D
N
F
30
I
C
M
L
20
H
A
B
E
G
10
0
15
10
5
0
5
10
15
Mean annual temperature (ºC)
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• The Rhizoshpere and Decomposition
• 40 photosynthetic dry matter fuels microbial
  growth in rhizosphere
• Fuels microbial growth which eventually releases
  nutrients ? increased availability to plants
  via mineralization
• Soil Microbial Loop

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Case Study Nutrient Cycling in the Hubbard Brook
Experimental Forest

• Vegetation strongly regulates nutrient cycling
• Research projects monitor ecosystem dynamics over
  long periods
• The Hubbard Brook Experimental Forest has been
  used to study nutrient cycling in a forest
  ecosystem since 1963

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Fig. 55-16a
(a) Concrete dam and weir
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Fig. 55-16c
80
Deforested
60
40
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Nitrate concentration in runoff (mg/L)
Completion of tree cutting
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Control
3
2
1
0
1965
1966
1967
1968
(c) Nitrogen in runoff from watersheds
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• Decomposition in Aquatic systems
• Impacted by environment
• Photosynthetic processes at surface
• Detritus falls to benthic zone
• Low oxygen, cool temperatures
• Stratification of water occurs through summer
• spring/fall turnover events ? mixing of water
  column

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• Export of resources ? loss of nutrients
• Example 1. logging
• Logs nutrients removed from forest
• Increased nutrient water flow ? nutrient loss via
  streams
• Stream salmon fishery
• Salmon represent nutrient transfer mechanism from
  sea to terrestrial ecosystem
• Fire
• Alters mineralization rate/processes
• Subsequent leaching/runoff ? nutrient loss

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• Import of nutrients/alteration of normal cycling
• Possible sources
• Runoff from adjacent ecosystems
• Agricultural
• Municipal/industrial
• Impacts
• Toxins some may accumulate higher levels food
  chain via bio-accumulation or bio-magnification
• Eutrophication nutrient enrichment in lake/pond
  ? biological activity ? stimulation of detrital
  food chain ? anoxia
• Global/environmental C,N and global climate

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• Two types of biogeochemical cycles based input
  source to ecosystems
• Sedimentary
• Rock and salt solution phases
• Include S, P
• Gaseous
• Global
• Include C, N, O
• Many cycles hybrid
• Exchange pool
• Reservoir

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• Carbon cycle
• Closely tied to energy flux
• Major exchange pool atm CO2 (at 0.03 )
• Uptake via photosynthesis
• Immobilized in carbonates of shells, fossil fuels
• Subject to daily seasonal flux

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• The Phosphorus Cycle
• Phosphorus is a major constituent of nucleic
  acids, phospholipids, and ATP
• Phosphate (PO43) is the most important inorganic
  form of phosphorus
• The largest reservoirs are sedimentary rocks of
  marine origin, the oceans, and organisms
• Phosphate binds with soil particles, and movement
  is often localized

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Fig. 55-14d
Precipitation
Geologic uplift
Weathering of rocks
Runoff
Consumption
Decomposition
Plant uptake of PO43
Plankton
Dissolved PO43
Soil
Uptake
Leaching
Sedimentation
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• Nitrogen cycle
• N essential to life amino acids, nucleic acids
• Atm. N2 stable, difficult bond to break
• Fixation largely biological (ca 90)
  agricultural use requires fossil fuel input

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• Fixation of N N
• Free living aerobics as Azotobacter, certain
  cyanobacter
• Lichen symbionants
• Mutualists associated with certain plant groups
• N2 N N (NH3)2
  NH4
• H energy
• 
  
  NO3

Ammonium form available to plants
Ammonia (gas)
Nitrate produced by soil bacteria from ammonium
may also be taken up by plants or mineralized to
N2
Under acidic conditions converts to ammonium but
may be lost to atmosphere
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• Organic nitrogen is decomposed to NH4 by
  ammonification, and NH4 is decomposed to NO3 by
  nitrification
• Denitrification converts NO3 back to N2

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Fig. 55-14c
N2 in atmosphere
Assimilation
Denitrifying bacteria
NO3

Nitrogen-fixing bacteria
Decomposers
Nitrifying bacteria
Ammonification
Nitrification
NH3
NH4
NO2


Nitrogen-fixing soil bacteria
Nitrifying bacteria
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Human activities now dominate most chemical
cycles on Earth

• As the human population has grown, our activities
  have disrupted the trophic structure, energy
  flow, and chemical cycling of many ecosystems

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Nutrient Enrichment

• In addition to transporting nutrients from one
  location to another, humans have added new
  materials, some of them toxins, to ecosystems

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Agriculture and Nitrogen Cycling

• The quality of soil varies with the amount of
  organic material it contains
• Agriculture removes from ecosystems nutrients
  that would ordinarily be cycled back into the
  soil
• Nitrogen is the main nutrient lost through
  agriculture thus, agriculture greatly affects
  the nitrogen cycle
• Industrially produced fertilizer is typically
  used to replace lost nitrogen, but effects on an
  ecosystem can be harmful

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Fig. 55-17
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Contamination of Aquatic Ecosystems

• Critical load for a nutrient is the amount that
  plants can absorb without damaging the ecosystem
• When excess nutrients are added to an ecosystem,
  the critical load is exceeded
• Remaining nutrients can contaminate groundwater
  as well as freshwater and marine ecosystems
• Sewage runoff causes cultural eutrophication,
  excessive algal growth that can greatly harm
  freshwater ecosystems

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Fig. 55-18
Winter
Summer