The nitrogen cycle

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The nitrogen cycle

• AWER 4500
• 20 Feb 2007

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Nitrogen

• Often a limiting nutrient for algal growth
• N can have a variety of electron states,
  therefore had lots of forms!
• Occasionally Toxic

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Types of Nitrogen

• N2 gas Very Abundant, mostly unavailable
• DIN, Dissolved inorganic nitrogen
• NH3 / NH4 Ammonia / Ammonium (nutrient, toxic at
  high levels)
• NO3- / NO2- Nitrate / Nitrite (nutrient, toxic
  at high levels)
• Organic nitrogen
• PON Particulate organic nitrogen (e.g. algae,
  bacteria, detritus)
• DON Dissolved organic nitrogen (proteins,
  tannins, etc. etc.)

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REDOX and nitrogen
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Sources of Nitrogen

• Precipitation
• Stream Inflow
• Nitrogen Fixation
• Lightening
• Biological

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N movement in a watershed
Precipitation, NO3, NH3
Watershed View of Nitrogen Transport Use
Nitrogen Fixation
N
NO3 NH3

• NO3- ions relatively mobile-washes N out of
  terrestrial environment
• In pristine environment, most nitrogen moves as
  DON (dissolved organic N)

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Biological N2 fixation

• Conversion of N2 gas to ammonium
• Very energetically expensive
• Needs
• Large supply of energy
• Nitrogenase enzyme
• Anoxic site
• Legumes, Cyanobacteria

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Cyanobacteria

• Common members of lakes and stream communities.
• Autotrophic
• Heterocysts
• Seasonal cycles

Heterocyst
Photos from Whitton and Potts 2000
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Nitrogen Fixation in Lake Titicaca, Peru
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Sources of Nitrogen

• Precipitation
• Stream Inflow
• Nitrogen Fixation
• Lightening
• Biological
• Anthropogenic (fertilizer)

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From Vitousek et al. 1997, Ecol. Appl.
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The nitrogen cycle
N2 gas
NITROGEN FIXATION
NH4
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The nitrogen cycle
N2 gas
NITROGEN FIXATION
NH4
R NH2
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Ammonia assimilation and ammonification

• Assimilation uptake conversion of ammonium to
  amino acids in algae bacteria
• Ammonification - conversion of organic N back
  to ammonia, by bacteria animals

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The nitrogen cycle
N2 gas
NITROGEN FIXATION
NH4
NO2-
NO3-
NITRIFICATION
R NH2
ASSIMILATION / MINERALIZATION
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Nitrification

• Aerobic reaction, using ammonia as an energy
  source
• Needs
• Ammonium
• Oxygen
• Nitrifying bacteria
• Loss of ammonia from water column

Summary 2 NH4 3 O2 ? 2 NO3-
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The nitrogen cycle
N2 gas
NITROGEN FIXATION
DNRA
NH4
NO2-
NO3-
NITRIFICATION
R NH2
ASSIMILATION / MINERALIZATION
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DNRA

• Dissimilatory nitrate reduction to ammonium
• Conversion of nitrate back to ammonium, carried
  out by bacteria
• Newly discovered, not well known

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The nitrogen cycle
N2 gas
NITROGEN FIXATION
DNRA
NH4
NO2-
NO3-
NITRIFICATION
NITRATE ASSIMILATION
R NH2
ASSIMILATION / MINERALIZATION
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Nitrate Assimilation and Reduction

• Assimilation - Conversion of nitrate to amino
  acids
• Requires reduction of nitrogen
• More energetically expensive than ammonium
  assimilation

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The nitrogen cycle
N2 gas
NITROGEN FIXATION
N2O
DENITRIFICATION
NO
DNRA
NH4
NO2-
NO3-
NITRIFICATION
NITRATE ASSIMILATION
R NH2
ASSIMILATION / MINERALIZATION
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The nitrogen cycle
N2 gas
NITROGEN FIXATION (make fertilizer)
N2O
DENITRIFICATION (burn organic matternitrate as
oxidizer)
NO
DNRA
NH4
NO2-
NO3-
NITRIFICATION burn ammonia for energy
(make building blocks Proteins)
(decompose proteins
NITRATE ASSIMILATION
(Uptake make Proteins)
R NH2
ASSIMILATION / MINERALIZATION
NH3NO3 match bomb (high energy compound
oxidizer Divine Strake
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Denitrification

• Nitrate as a terminal electron receptor, in
  absence of oxygen
• Sugars burned using nitrate as oxidizing source
  instead of O2
• Nearly as efficient as aerobic respiration
• Requirments
• Anoxia
• Source of nitrate
• Source of carbon
• Denitrifying bacteria

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The nitrogen cycle
N2 gas
NITROGEN FIXATION
N2O
DENITRIFICATION
NH3 gas
NO
VOLATIZATION
DNRA
NH4
NO2-
NO3-
NITRIFICATION
NITRATE ASSIMILATION
R NH2
ASSIMILATION / MINERALIZATION
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Seasonal Nutrient CyclesSummerOligotrophic Lakes

• Oxic Epilimnion
• Oxic Hypolimnion

Temp
O2
Summer Stratification
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N in oligotrophic lakes

• NH3 NO3 assimilation in epilimnion
• Ammonification in epilimnion
• Nitrification in epilimnion and hypolimnion
• Mineralization in hypolimnion epi- and
  hypolimnetic sediments

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Seasonal Nutrient CyclesSummerEutrophic Lakes

• Oxic Epilimnion
• Anoxic Hypolimnion

Temp
O2
Summer Stratification
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N in eutrophic lakes

• Ammonification in epilimnion
• Mineralization in hypolimnion epi-
  hypolimnetic sediments
• Nitrification in epilimnion, but not hypolimnion
• Denitrification in hypolimnion
• NH4 diffusion from sediments

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