Nutrient Cycles

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Nutrient Cycles

• Nutrient requirements
• Biogeochemical cycles
• Rates of decomposition
• Plant adaptations in low nutrient conditions

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Nutrient Requirements for Plant Growth

• Taken up in gaseous form, Oxygen (O2), Carbon
  CO2, and from roots - Water (H2O).
• Derived from water and carbon dioxide
• Rest are taken up from soil solutions
• Macro-nutrients Nitrogen (N), Phosphorous (P),
  Potassium (K),
• Calcium (Ca), Magnesium (Mg), Sulfur (S)
• Micro-nutrients Boron (B), Copper (Cu), Iron
  (Fe), Manganese (Mn), Molybdenum (Mo), Zinc (Zn)

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Nutrient Cycles

• Nutrient requirements
• Biogeochemical cycles
• Rates of decomposition
• Plant adaptations in low nutrient conditions

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Biogeochemical Cycling

• The cycling of nutrients through ecosystems via
  food chains and food webs, including the exchange
  of nutrients between the biosphere and the
  hydrosphere, atmosphere and geosphere (e.g.,
  soils and sediments)

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• Ecosystems produce and process energy primarily
  through the production and exchange of
  carbohydrates which depends on the carbon cycle.
• Once energy is used, it is lost to the ecosystem
  through generation of heat
• Carbon is passed through the food chain through
  herbivory, predation, and decomposition, it is
  eventually lost to the atmosphere through
  decomposition in the form of CO2 and CH4 . It is
  then re-introduced into the ecosystem via
  photosynthesis.
• However, the amount of carbon present in a system
  is not only related to the amount of primary
  production, as well herbivory and predation
  (e.g., secondary production), it is also driven
  by the rates of decomposition by micro-organisms
• Atmospheric carbon is rarely limiting to plant
  growth

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• When we look at other nutrients, a somewhat
  different picture emerges than with the energy
  cycle e.g., phosphorous in a food chain within
  a small pond.
• Algae remove dissolved phosphorous from the water
• The phosphorous is then passed through different
  trophic levels through herbivory and predation.
• At each level there is some mortality, and then
  the phosphorous is passed to decomposers
• These organisms release phosphorous into the
  water where it is again taken up by primary
  producers and the whole cycle starts up again

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Key Elements of Biogeochemical Cycles

• Where do the nutrients that ecosystems use come
  from?
• What happens to the nutrients within the
  ecosystem itself?
• What happens to the nutrients once they leave the
  ecosystem?
• Once nutrients are cycled through an ecosystem,
  how do they get back?
• What are the rates of exchange of nutrients
  between the different pools?

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Nutrient Pools and Nutrient Flux

• Nutrient pool a specific component or
  compartment where a nutrient resides
• Can be a single organism, a population, a
  community, a trophic level, and an abiotic
  feature (e.g., lake, soil, atmosphere, etc.)
• Nutrient flux the rate of exchange (e.g., unit
  of material per unit time) of nutrients between
  pools

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• Example of changes in the amounts of tracer
  phosphorous being exchanged within an aquatic
  food web
• The values themselves represent changes in the
  pool levels, where each one of the lines
  represents a different pool
• Understanding the feeding relationship allows us
  to build a nutrient cycle model for this ecosystem

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• Model of phosphorous cycle for an aquatic
  ecosystem flux rates per day shown.
• This system is not closed inputs, probably from
  run-off from land.
• Exports include ? herbivores moving outside of
  system and dead plant/animal material moving out
  of system, probably through sedimentation.
• Rate of uptake by plants is directly proportional
  to net primary production.
• Exchange of nutrients by higher trophic levels is
  controlled by processes regulating secondary
  production.
• Rates of inputs and outputs of nutrients from an
  ecosystem are driven by both biotic and abiotic
  factors.

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Types of Biogeochemical Cycles

• Three major categories of biogeochemical cycles
  based on slowest-changing pool(reservoir)
• Gaseous cycles of C, O, H20
• Gaseous cycle of N, (S)
• Sedimentary cycles of the remaining nutrients

Global scale
Local scale
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Sedimentary Cycles
Gaseous Cycles
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Major Components of Nitrogen Cycle
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Biological Nitrogen Fixers

• Cyanobacteria blue-green algae
• Free living soil bacteria
• Mycorrhizae
• Symbiotic bacteria living in root nodules

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Root nodules on ? Cassia fasciculata
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NO from lightning
Lightning N2 O2 ? NO O2 ? Nitrate (NO3)
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Phosphorous Cycle
Phosphate PO4-3
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Potassium
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Sources of Nutrients
Atmosphere
Parent Material
Run-off, Ground water
Floods
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Nutrient Cycles

• Nutrient requirements
• Biogeochemical cycles
• Rates of decomposition
• Plant adaptations in low nutrient conditions

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Simple Model of Soil Decomposition/ microbial
respiration
H2O, O2
CO2 or CH4
Litter
Energy
Microbial Population
Organic Soil
Nutrients
Dissolved Nutrients
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Factors Controlling Microbial Respiration

• Availability of oxygen
• CO2 versus CH4 production
• Temperature
• Moisture
• Quality of material comprising dead organic matter

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Simple Model of Simple Model of Soil
Decomposition/ microbial respiration
H2O, O2
CO2 or CH4
Litter
Energy
Microbial Population
Organic Soil
Nutrients
Dissolved Nutrients
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k is the fraction of a material that decomposes
in a given year
Decomposition as a Function of Lignin Content
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Residence Time

• Residence time is the length of time it takes for
  biomass or a nutrient to be completely decomposed
  or recycled from the forest floor

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Residence times
Coniferous forests have longer residence times
than deciduous ? C/N control Boreal forests
have longer residence times than temperate
forests ? temperature control
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Nutrient Cycles

• Nutrient requirements
• Biogeochemical cycles
• Rates of decomposition
• Plant adaptations in low nutrient conditions

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Tree Nutrient Content
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Translocation of Nutrients

• Prior to shedding leaves in the fall,
  translocation of nutrients often takes place in
  trees
• This allows tree to retain essential nutrients
  that are hard to come by
• Spruce trees remove more nutrients than other
  coniferous trees
• An adaptation to poor nutrient sites

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Question do plants growing on sites with low
soil nutrients have low nutrient contents as well?

• The answer is no
• Plants on sites with low nutrients tend to have
  higher nutrient contents
• They have a higher nutrient use efficiency

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Nutrient Use Efficiency (NUE)

• Some plants are more efficient at using nutrients
  because it gives them selective advantages in low
  nutrient conditions
• NUE A / L
• A the nutrient productivity (dry matter
  production per unit nutrient in the plant)
• L nutrient requirements per unit of plant
  biomass

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A common pattern found in ecosystem productivity
is saturation curve. Productivity increases
linearly with N availability, up to a certain
point, when other resources become limiting
(e.g., light, water, temperature, other nutrients)
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• Three types of relationships with respect to
  limitations of nutrients
• Production is independent of resource
  availability
• Production is a linear function of resource
  availability
• At some point, another resource becomes limiting

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Factors Influencing Nutrient Availability

• Presence of nitrogen fixers
• Microbial activity
• Fire
• Precipitation patterns
• Soil drainage
• Soil temperature, moisture

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H2O - Precipitation
CO2
Fire
GHG
Photosynthesis
Aeolian, Atmospheric Deposition
Internal translocation
N2, O2
Litterfall
nutrients
N fixers
CH4, CO2
Organic soil
Through-fall nutrients
Dissolved nutrients
Nutrients
Energy, Nutrients
Upper mineral soil
Microbes
Lower mineral soil
Leaching, run off
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Boreal forest has the largest available nutrient
pool in soil, but lowest rates of production,
where as tropical forest has lowest soil pool,
and highest production.
 
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Role of Disturbances in Nutrient Cycling

• Type of disturbance important
• Fire versus logging versus large-scale mortality
• Disturbances directly alter biotic and abiotic
  controls on nutrient cycling
• Rates of primary production
• Controls on evapotranspiration
• Influences on surface runoff
• Soil temperature/moisture ? decomposition rates
• Linkages between terrestrial/aquatic systems

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Hubbard Brook watershed, upstate New Hampshire.
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Nutrient Cycles

• Nutrient requirements
• Biogeochemical cycles
• Rates of decomposition
• Plant adaptations in low nutrient conditions

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Upland White Spruce Succession
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Nutrient Cycling in Upland White Spruce