Nutrient pump (temperate lake turnover)

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Nutrient pump (temperate lake turnover)
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• BIOGEOCHEMICAL CYCLES
• A few general points (terrestrial systems)
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• Nutrient cycling is never perfect i.e. always
  losses from system

Inputs
Outputs
Precipitation Runoff stream flow
Particle fallout from atmosphere Wind loss
Weathering of substrate Leaching
Fertilizer pollution Harvesting
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terrestrial systems contd

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

(i.e. relatively tight cycling is the norm)
3. Disturbances (e.g. deforestation) often
uncouple cycling
4. Gradient in rates of decomposition and
nutrient cycling 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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• Deforestation is a major change in community
  structure, with a consequent
• loss of nutrients (Krebs Fig 27.7 p567)
• x20-30 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
POLAR TROPICS
Decomposition Slow Rapid
Proportion nutrients in living biomass Low (mostly in dead organic matter) High
Cycling Slow Rapid

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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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Litter accumulation in forest floor
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Relationship between rate of litter decomposition
and litter quality (C/N ratio)
Plant species weight loss in 1 year C/N ratio bacterial colonies fungal colonies Bact / Fungi ratio
Mulberry 90 25
Redbud 70 26
White Oak 55 34
Loblolly pine 40 43
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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micro
meso
macro
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
Plant species weight loss in 1 year C/N ratio bacterial colonies fungal colonies Bact / Fungi ratio
Mulberry 90 25 698 2650 264
Redbud 70 26 286 1870 148
White Oak 55 34 32 1880 17
Loblolly pine 40 43 15 360 42
Faster decomposition at higher bact/fungi ratios