Nutrient cycling

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Nutrient cycling Ecosystem Health

• READINGS for this lecture series
• KREBS chap 27. Ecosystem Metabolism III
  Nutrient Cycles
• KREBS chap 28. Ecosystem Health
• Human Impacts Pp 590 600
• WEB Downloads

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NUTRIENT CYCLING

• Energy 1-way flow
• - eventually gets lost
• Nutrients cycle

mineralization
Inorganic (rocks, air, water)
Organic (living organisms)
assimilation
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• Two main types of cycles
• 1. Biochemical cycles
• Redistribution within an individual organism
• This relates to r- and K-selection (Biol 303)
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• 2. Biogeochemical cycles
• Local - exchange occurs within and between
  terrestrial/aquatic ecosystems
• Global exchange occurs between atmosphere
  and terrestrial/aquatic ecosystems

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• Two main types of cycles
• 1. Biochemical cycles
• Redistribution within an individual organism
• This relates to r- and K-selection (Biol 303)
•  
• 2. Biogeochemical cycles
• Local - exchange occurs within and between
  terrestrial/aquatic ecosystems
• Global exchange occurs between atmosphere
  and terrestrial/aquatic ecosystems
• e.g. CO2, SO2, NOx

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Krebs Fig. 27.12 p573
SULPHUR CYCLE
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Krebs Fig. 28.8 p591
CARBON CYCLE
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Krebs Fig. 27.17 p579
NITROGEN CYCLE
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78 of air
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• These figures have
• All sorts of rates of transfer
• We can compare between systems

• More interesting
• What influences the rates?
• What are the impacts of altering the rates?

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• These figures have
• All sorts of rates of transfer
• We can compare between systems

• More interesting
• What influences the rates?
• e.g. forms of nutrients, types of organisms
• What are the impacts of altering the rates?
• e.g. disturbance, pollution, etc.

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Compartment Models

• Quantitative descriptions of storage and
  movement of nutrients among different
  compartments of an ecosystem
• Coarse few broad compartments
• e.g. plants, herbivores
• Fine many detailed compartments
• e.g. separate species

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Compartment Models

• POOL the quantity of a particular nutrient in
  a compartment
• FLUX the quantity moving from one pool to
  another per unit time
• TURNOVER TIME the time required for movement
  of an amount of nutrient equal to the quantity
  in the pool (POOL/FLUX)

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Krebs Fig. 27.2 p562
Phosphorus cycle in a lake (simplified)
Turnover time (water) 9.5 (pool) /152 (flux)
0.06 day
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NUTRIENT PUMP

• Any biotic or abiotic mechanism responsible for
  continuous flux of nutrients through an ecosystem

• Biotic tree roots, sea birds,
• Pacific salmon
• Abiotic lake overturn, ocean upwelling

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Nutrient pump (Terrestrial)
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Mycorrhizae
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Mycorrhizae
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CEC Cation Exchange Capacity
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Nutrient pumps (Marine)
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Nutrient pump (temperate lake turnover)
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• BIOGEOCHEMICAL CYCLES
• A few major points (general principles)
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• Nutrient cycling is never perfect i.e. always
  losses from system
• input and output (terrestrial systems)

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terrestrial systems contd

• Inputs and outputs are small in comparison
• to amounts held in biomass and recycled

3. Relatively 'tight' cycling is the norm

• Disturbances (e.g. deforestation) often uncouple
  cycling

5. Gradient from poles to tropics
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HUBBARD BROOK FOREST

• Experiments done to
• Describe nutrient budget of intact forest
• Assess effects of logging on nutrient cycles

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Annual Nitrogen budget for the undisturbed
Hubbard Brook Experimental Forest. Values are
Kg, or Kg/ha/yr
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• Disturbances (e.g. deforestation) often
  uncouples cycling, and a consequent
• loss of nutrients (Krebs Fig 27.7 p567)
• x13 normal loss of NO3 in Hubbard Brook
• reduction in leaf area
• 40 more runoff (would have transpired)
• more leaching
• more erosion, and soil loss
• decouples within-system cycling of decomposition
  and plant uptake processes
• all the activities (and products) of spring
  decomposition get washed away

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Logging causes decoupling of nutrient cycles and
losses of nitrogen as nitrates and nitrites
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Calcium
Concentrations of ions in streamwater from
experimentally deforested, and control,
catchments at Hubbard Brook.
Potassium
Nitrate-N
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Uncoupling of N-cycle
1) Logging causes increased nitrification
2) H displace nutrient cations from soil micelles
H gtCagtMggtKgtNa
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5. Gradient from poles to tropics

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Relative proportion of Nitrogen in organic matter
components
ROOTS
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Relative proportion of Nitrogen in organic matter
components
SHOOTS
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DECOMPOSITION

• IF TOO SLOW
• Nutrients removed from circulation for long
  periods
• Productivity reduced
• Excessive accumulations of organic matter (e.g.
  bogs)

• IF TOO FAST
• Nutrient depletion
• Poor chemistry and physics of soil (e.g.
  decreased soil fertility, soil moisture and
  resistance to erosion) (e.g. tropical laterites)

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• WHAT DETERMINES DECOMPOSITION RATES IN FORESTS?
• moisture and temperature
• pH of litter and the forest floor
• more acid promotes fungi, less bacteria
• species of plant producing the litter
• chemical composition of the litter
• C/N ratio - high gives poor decomposition
• microbes need N to use C
• N often complexed with nasties (e.g. tannin)
• optimum is 251
• Douglas fir wood 5481
• Douglas fir needles 581
• alfalfa hay 181
• activities of soil fauna e.g. earthworms

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• Decomposition Rates influenced by
• temperature
• moisture
• pH, O2
• quality of litter
• soil type (influences bugs)
• soil animals
• type of fauna / flora
• rapid if bacterial
• slow if fungal

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• RATE OF DECOMPOSITION
• humid tropical forests about 2 - 3 weeks
• temperate hardwood forests 1 - 3 years
• temperate / boreal forests 4 - 30 yr
• arctic/alpine / dryland forests gt40 years
• generally, rate of decomposition increases
  with increased amount of litterfall

Residence time the time required for the
complete breakdown of one years litter fall
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Residence times (years)
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Residence times (years)
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• Decomposition Rates influenced by
• temperature
• moisture
• pH, O2
• quality of litter
• soil type (influences bugs)
• soil animals
• type of fauna / flora
• rapid if bacterial
• slow if fungal

(mineral content, C/N ratio)
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Relationship between rate of litter decomposition
and litter quality (C/N ratio)
Faster decomposition at lower C/N ratios
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• Decomposition Rates influenced by
• temperature
• moisture
• pH, O2
• quality of litter
• soil type (influences bugs)
• soil animals
• type of fauna / flora
• rapid if bacterial
• slow if fungal

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100 90 80 70 60 50 40 30 20 10 0
0.5 mm mesh bags
leaf litter remaining
7.0 mm mesh bags
(J) J A S O N D J F M A
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Litter decomposers
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• Decomposition Rates influenced by
• temperature
• moisture
• pH, O2
• quality of litter
• soil type (influences bugs)
• soil animals
• type of fauna / flora
• rapid if bacterial
• slow if fungal

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Relationship between rate of litter decomposition
and the balance between bacteria and fungi
Faster decomposition at higher bact/fungi ratios
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