Nitrogen and Phosphorus Cycles

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Nitrogen and Phosphorus Cycles

• Martha E. Rosemeyer
• IES
• January 29, 2003

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One of three dynamic processes of ecosystems

• Energy flow
• Nutrient cycles
• Succession

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Energy flows from the sun to entropy through a
functioning food web
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Two types of nutrient cycles

• Volatile
• atmospheric reservior
• N, S
• Non-volatile
• No atmospheric reservior
• P, K, Ca, Mg all micronutrients

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Nitrogen cycle
N2O N2
Gliessman, 1998 modified
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Nitrification
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N Pool Size (Stocks) are relatively big

• Atmosphere g N 1021
• Terrestrial biomass g N 1015
• Soil organic matter g N 1015

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Flows small Rate limiting steps with respect to
getting N into terrestrial system

• Via lightning (very small)
• Via Biological N Fixation
• natural
• legume crops
• Via Industrial fixation of fertilizer
• Via Fossil fuel burning

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What is biological nitrogen fixation (BNF)?

• Second most important biochemical reaction of
  ecosystem after photosynthesis to life on earth
• Involves the reduction of N from molecular state
  N2 to NH3 in which form it can be incorporated
  into an organic (C containing) molecular
  structure
• Involves microbes

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BNF

• Regardless of organism uses nitrogenase enzyme
  for to fix N2 to ammonia NH3
• NH3 org. acids amino acids proteins

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White clover, Trifolium repensTrebol blanco
Temperate zone green manure crop
seed
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Nitrogen cycle anthropogenic emissions
Fossil fuel burning
Fertilizer
N2O N2
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Gliessman 1998 modified
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Human-caused NF or greater Natural
terrestrial BNF

• BNF from crops 40 x 1012 g N fixed/yr
• Fertilizer industry 80
• Fossil fuel burning 20
• Total human-caused 144
• BNF terrestrial ecosystems 100

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Microbes The Ancient Ones

• Responsible for all major processes on earth,
  including decomposition and photosynthesis and
  nutrient cycling
• Major cycles of Earth could continue without
  plants and animals
• Most microbes are beneficial!

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Most bacteria 99 cannot be cultured

• Therefore we know little about them!

A ton of microscopic bacteria may be active in
each acre of soil. Bacteria dot the
surface of strands of fungal hyphae. --Soil
Biology Primer
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Life on earth starting with microbes

• First N fixed by lightning and meteor impact
• Formation of first amino acids and protein
• N-fixation early because N-limited environment in
  water
• Terrestrial system P limited environment
• Mycorrhizal fungi have allowed plants to become
  terrestrial

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Nitrogen cyclemediated by microbes
Microbial mediation
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Gliessman 1998 modified
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P Cycle

• P is an important plant nutrient, anion
• Released from soil, taken up by plants and
  released to soil
• Green Rev ag is totally dependent on P rock
• P rock is basically a valuable, non-renewable
  resource for agriculture
• some deposits contaminated with Cd, Pb, As
• 10 of the P which is applied to land is flushed
  way causing eutrophication of lakes

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Dead zone in Gulf of Mexico due to nutrients from
Mississippi
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The phosphorus cycle
Microbial mediation
mycorrhizae
Gliessman 1998
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P cycle
Brady, 1999
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Roots of Common Bean (Phaseolus vulgaris) from
Costa Rica
Mycorrhizal fungus stained blue
Non mycorrhizal
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Components of association

• fungus root mycor rhiza
• They are the rule, not the exception
• Most of worlds vascular plants have
  association, except Brassicaceae (cabbage family)
  and a few other plant families
• Fungus takes up water and P, receives CHO from
  plant
• Enables plants to live in low P, low water
  environments

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Mycorrhizae aid soil aggregation
Mycorrhizal fungi link root cells to soil
particles. In this photo, sand grains are bound
to a root by hyphae from endophytes (fungi
similar to mycorrhizae), and by polysaccharides
secreted by the plant and the fungi. Credit
Jerry Barrow, USDA-ARS Jornada Experimental
Range, Las Cruces, NM.
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Closing nutrient cycles

• Making the cycle circular
• The nutrient comes back to the stock or pool
  that it started with-- none is lost
• In a human time frame
• This is an important component of sustainability

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Chinese have used night soil for centuries to
close nutrient cycle