The Nitrogen Cycle

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The Nitrogen Cycle

• Aquatic Ecology

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Forms of Nitrogen

• Urea ? CO(NH2)2
• Ammonia ? NH3 (gaseous)
• Ammonium ? NH4
• Nitrate ? NO3
• Nitrite ? NO2
• Atmospheric Dinitrogen ?N2
• Organic N

3
Nitrogen History

• More money and effort are spent on the
• management of N and S than any other
• mineral nutrient
• Deficiencies are world wide, in cultivated
• and natural environments
• Excesses cause a degradation of the
• quality of live, N pollution

4
Nitrogen History

• Adding N to soils is one of the most
• costly parts or agriculture

5
Significance of N

• Nitrogen (N) is an essential component of
• DNA, RNA and proteins, the building blocks
• of life
• All organisms require nitrogen to live and
• grow
• Although the majority of the air we breathe is
• N2, most of the nitrogen in the atmosphere is
• unavailable for use by organisms
• This is because the strong triple bond
• between the N atoms in N2 molecules makes
• it relatively inert

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Significance of N

• Nitrogen is an incredibly versatile
• element, existing in both inorganic and
• organic forms as well as many different
• oxidation states
• The movement of nitrogen between the
• atmosphere, biosphere and geosphere
• in different forms is described by the
• nitrogen cycle

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Significance of N

• N2 gas must first be converted to more a
• chemically available form such as ammonium
• (NH4), nitrate (NO3-), or organic nitrogen
• (e.g. urea - (NH3)2CO)
• The inert nature of N2 means that biologically
• available nitrogen is often in short supply in
• natural ecosystems, limiting plant growth and
• biomass accumulation

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Chemistry of N
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Chemistry of N

• The valence range which N undergoes
• in its biogeochemical cycling is full
• Going from loss of all five of its outer shell
• electrons (5) to other elements
• To the gain of three electrons from other
• elements (-3) to complete all of the orbitals
• of its outer electron shell

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Chemistry of N

• On the right-hand side of the depicted N
• cycle, the N atom can eventually lose all
• five of its outer shell electrons to O
• With this, N can eventually become fully
• oxidized as nitrate (NO3-)

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Chemistry of N

• On the left-hand of the depicted N cycle, N
• can eventually add three electrons to fill all of
• its outer shell electron orbitals from elements
• such as hydrogen (H) and carbon (C)
• With such gain of electrons, N can be fully
• reduced to ammonia (NH3) - which most
• commonly exists in its ionic form, ammonium
• (NH4)
• Or N can be fully-to-partially reduced in
• organic compounds

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Nitrogen Cycle - Animation
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6 Important Processes

• Nitrogen fixation
• Nitrogen uptake/Assimilation
• Nitrogen mineralization
• Nitrification
• Denitrification
• Volatilization

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Nitrogen Fixation
N2
N2O
NH4
NO2
R-NH2
NO
NO2
NO3
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Nitrogen fixation

• N2 ? NH4
• N2 is converted to ammonium
• Essential because it is the only way that
• organisms can attain nitrogen directly from the
  atmosphere
• Energy intensive process
• N2 8H 8e- 16 ATP 2NH3 H2 16ADP 16
  Pi

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Nitrogen fixation

• Certain bacteria, Rhizobium, are the only
• organisms that fix nitrogen through metabolic
• processes
• N fixing bacteria often form symbiotic
• relationships with host plants (e.g. beans,
• peas, and clover)
• N fixing bacteria inhabit legume root nodules
• and receive carbohydrates and a favorable
• environment from their host plant in exchange
• for some of the nitrogen they fix

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N fixation w/ Blue-Green Algae

• In aquatic environments, blue-green algae (really
  a bacteria called cyanobacteria) is an important
  free-living nitrogen fixer

Anacystis bloom Ford Lake August 2002.
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Measuring water transparency with a Secchi disk
on Ford Lake during a bloom of Aphanizomenon.
August 2004.
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Nitrogen fixation in Cyanobacteria

• Nitrogen fixation occurs in special cells called
  heterocysts, but
• Not all cyanobacteria have heterocysts or can fix
  nitrogen
• Some cyanobacteria can fix nitrogen without
  heterocysts

www.bio.purdue.edu/people/faculty/sherman/ShermanL
ab
www.micrographia.com/specbiol/bacteri/bacter/bact0
200/anabae03.jpg
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N Fixation Contd

• ½ can be contributed by N-fixing org.
• The rest comes from atmospheric deposition
  (lightning) or runoff.
• Salmon
• Alders

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Nitrification
N2
N2O
NH4
NO2
R-NH2
NO
NO3
NO2
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Nitrification

• NH4 ? NO3- or NO2
• Some of the ammonium produced by
• decomposition is converted to nitrate via a
• process called nitrification Nitrosomonas and
  Nitrobacter
• The bacteria that carry out this reaction gain
• energy
• Requires the presence of oxygen
• circulating or flowing waters and the very
  surface
• layers of soils and sediments

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Methemoglobinemia

• Nitrate is one of the most common groundwater
  contaminants in rural areas.
• It is regulated in drinking water primarily
  because excess levels can cause
  methemoglobinemia, or "blue baby" disease.
• Affects nursing infants b/c gut is too acidic (pH
  2) for denitrifying bacteria to reduce nitrate to
  nitrite.
• Nitrite combines w/ hemoglobin to produce
  methemoglobin, does not break down easily or
  carry Oxygen.

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Methemoglobinemia

• Nitrate in groundwater originates primarily from
  fertilizers, septic systems, manure storage or
  spreading operations.
• Fertilizer nitrogen that is not taken up by
  plants, volatilized, or carried away by surface
  runoff leaches to the groundwater in the form of
  nitrate (nitrification NH4 ? NO3).
• This not only makes the nitrogen unavailable to
  crops, but also can elevate the concentration in
  groundwater above the levels acceptable for
  drinking water quality.
• Nitrogen from manure similarly can be lost from
  fields, barnyards, or storage locations.
• Septic systems also can elevate groundwater
  nitrate concentrations because they remove only
  half of the nitrogen in wastewater, leaving the
  remaining half to percolate to groundwater.

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Denitrification
N2
N2O
NH4
NO2
R-NH2
NO
NO2
NO3
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Denitrification

• NO3- ? N2 ? N2O
• Oxidized forms of nitrogen such as nitrate and
  nitrite (NO2-) are converted to dinitrogen (N2)
  and, to a lesser extent, nitrous oxide gas
• An anaerobic process that is carried out by
  denitrifying bacteria, which convert nitrate to
  dinitrogen in the following sequence
• NO3- ? NO2- ? NO ? N2O ? N2.

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Denitrification

• Effluent of sewage treatment plants.
• Denitrification by bacteria converts
  nitrogen-oxygen compounds into nitrogen gas.
• N leaves treatment plant as a gas to reduce the
  amount of DIN in effluent.
• USBF (Upflow Sludge Blanket Filtration)

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N Uptake/Assimilation

• NH4 ? Organic N
• The ammonia produced by nitrogen fixing
• bacteria is usually quickly incorporated into
• protein and other organic nitrogen
• compounds, either by a host plant, the
• bacteria itself, or another soil organism
• When organisms nearer the top of the food
• chain eat, they are using nitrogen that has
• been fixed initially by nitrogen fixing bacteria

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Ammonification or Mineralization
N2
N2O
NH4
NO2
R-NH2
NO
NO2
NO3
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Ammonification/Mineralization

• Organic N ? NH4
• After nitrogen is incorporated into organic
• matter, it is often converted back into
• inorganic nitrogen,
• During this process, usually called decay, a
• significant amount of the nitrogen contained
• within the dead organisms is converted to
• ammonium
• Once in the form of ammonium, nitrogen is
• available for use by plants

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Ammonia Volatilization

• The process of nitrogen loss as ammonia gas
  from urea forms under alkaline conditions
• During this process, ammonium is converted into
  NH3 gas which is then lost to the air
• In cooler conditions the enzyme breaks down
  urea much slower
• Thus, little ammonia gas is lost when urea is
  applied to cool soils
• Urea may originate from animal manure, urea
  fertilizers and, to a lesser degree, the decay of
  plant materials.

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Nitrogen Excretion

• N compounds are metabolized by animals for energy
  NH3 is a waster product.
• If O2 is present oxidized to NO3 or NO4
• If O2 is absent NH3 will accumulate.
• Aquatic Snails N is excreted by diffusion of
  (highly toxic) ammonia NH3 into the water.
• Terrestrial Snails excrete N as cyclic C-N
  compounds (uric acid) b/c NH3 cannot be easily
  washed away. NH3 would poison their lungs.

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Nitrogen Excretion

• Urea HN(CH3)2 - Excreted if animal is too large
  to excrete NH4 water is available.
• Less toxic than NH4
• Fishes excrete waste as urea

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Nitrogen Cycle in Lakes
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Aquarium N Cycle
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Tank Cycling

• New System Cycle tank using live rock
• If the ph is under 7, you will have ammonium. If
  the ph is 7 or higher you will have ammonia.
• Nitrates are not as harmful to tropical fish as
  ammonia or nitrites, but nitrate is still harmful
  in large amounts.
• Tropical Tank Setup Website