Nutrient Cycles

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Nutrient cycles
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Ecosphere Photo
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Earth Photo
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Nutrient cycles

• Nutrient cycles, or biogeochemical cycles,
  involve natural processes that recycle nutrients
  in various chemical forms in a cyclic manner from
  the non-living environment to living organisms
  and back to the non-living environment again
• Types of nutrient cycles
• Hydrologic cycle
• Atmospheric cycles
• Sedimentary cycles

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The water cycle
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• No water, no life
• determines ecosystem structure water-living
  (aquatic) communities important for supporting
  life on land
• affects nutrient availability
• Evaporation and transpiration lead to
  condensation, to precipitation, to percolation
  and runoff, and all over again
• Powered by energy from the sun and gravity
• 84 of water vapor from the oceans (71 of the
  Earths surface)
• 77 of precipitation falls back into the sea
• Some precipitation locked in glaciers

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• runoff, erosion, moves soil and weathered rock
• primary sculptor of the earths landscape
• dissolves many nutrient compounds, transporting
  nutrients
• Percolation dissolves minerals and moves them
  into groundwater
• Times for water to cycle through various
  pathways
• Water table 300-4600 years Lakes 13 years
  Streams 13 days Atmosphere 9 days Ocean
  37,000 years Glaciers 16,000 years
• Evaporation natural distillation also purified
  by chemical and biological processes in the soil
• Hydrologic, atmospheric or sedimentary?

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The carbon cycle
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• Essential to life
• Basic building block of carbohydrates, fats,
  proteins, nucleic acids and all other organic
  compounds
• CO2 is a heat-trapping greenhouse gas regulates
  heat, with major impacts on ecosystem function
• Cycling times for CO2 Atmosphere, 3 years
  Soil, 25-30 years Oceans, 1,500 years
• Hydrologic, atmospheric or sedimentary?

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The phosphorous cycle
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• essential nutrient of plants and animals, used in
  DNA, nucleic acids, fats, cell membranes, and
  bones, teeth and shells
• from phosphate deposits on land and shallow ocean
  sediments to living organisms and slowly back to
  the land and ocean
• Very little in the atmosphere, only as small
  particles of dust
• much more rapidly through living components than
  through geological formations animals get by
  eating producers or animals that eat producers
• Animal wastes and decay return much of this
  phosphorous to the soil, streams, and eventually
  to ocean bottom and into rock cycle
• Hydrologic, atmospheric or sedimentary?

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The nitrogen cycle
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• Nitrogen is necessary for vital organic compounds
  such as amino acids, proteins, DNA and RNA
• In short supply in both terrestrial and aquatic
  ecosystems
• N2 78 of the volume of the troposphere
• Cannot be directly used by organisms
• Must be converted to compounds that can enter
  food webs by the process of nitrogen fixation

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• Nitrogen fixation
• Specialized bacteria convert N2 to ammonia (NH3)
  by the reaction
• N2 3H2 2NH3
• Cyanobacteria in soils and water, and Rhizobium
  bacteria in small nodules in legume root systems
• Nitrification NH3 converted by specialized
  aerobic nitrite (NO2-), toxic
• Converted to nitrate (NO3-) ions, which are
  easily taken up by plants as nutrients

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• Nitrogen fixation
• Assimilation NO3- taken up by plants and used
  to make nitrogen-containing organic molecules
• Animals get nitrogen by eating plants or
  plant-eating animals
• Decomposers convert to NH3 and ammonium (NH4)
  ammonification
• Denitirification - specialized bacteria convert
  NH3 and NH4 back into NO2- and NO3, and then to
  N2 and N2O, released into the atmosphere

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• Easily leached by water, limiting productivity
  potential.
• Hydrologic, atmospheric or sedimentary?

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Back to the Ecosphere
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How do humans affect nutrient cycles?
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• Water cycle
• Drain fresh water from streams, lakes, and
  underground sources
• Clear vegetation increasing runoff, reducing
  infiltration, increasing erosion and risk of
  flooding
• Modify water quality by adding nutrients
  (phosphates) and changing ecological processes
  that naturally purify water

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• Carbon cycle
• Put more CO2 in the atmosphere than plants can
  remove
• Deforestation reduces the amount of vegetation to
  remove CO2
• Burning fossil fuels and wood releases more CO2
  than natural processes
• What happens when we have to much heat-trapping
  gas?

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• Phosphorous cycle
• Mine large phosphate rock for fertilizers and
  detergents
• Cutting tropical forests little phosphorous in
  soil, all bound up in organic matter which
  usually rapidly recycles but we remove the
  biomass or burn it, allowing it to be rapidly
  washed away by runoff, leaving the land
  unproductive
• Add excess phosphate to aquatic ecosystems in
  runoff from agricultural operations, causing
  explosive plant growth creating surface mats
  which block sunlight dying plants feed bacteria
  which uses up most of the oxygen in the water.

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• Nitrogen cycle
• Emit nitric oxide (NO) when burning fuels leads
  to acid rain
• Emit heat-trapping nitrous oxide (NO2) into the
  atmosphere
• Remove nitrogen from the earths crust for
  fertilizers, harvesting nitrogen-rich biomass,
  and increase leaching through irrigation
• Remove nitrogen from topsoil when burning
  grasslands and clearing forests also emits
  nitrous oxides
• Add excess through runoff and sewage promotes
  overgrowth of algae, which dies, breaks down, and
  decomposition by bacteria depletes the water of
  oxygen disrupts aquatic systems reduces aquatic
  biodiversity
• Add excess nitrogen to atmosphere allowing weedy
  plants to outcompete other plants, reducing
  biodiversity

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Experimental impacts on nitrogen cycling in a
disturbed habitat
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Nitrogen cycles in an experimental ecosystem
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