Global Nitrogen Cycle

1
Global Nitrogen Cycle
SOS 6456March 28, 2006

• Nitrogen chemistry (forms and characteristics)
• Biological N transformations
• Anthropogenic N fixation
• Atmospheric chemistry
• Global N cycle
• Human impacts
• Eutrophication of aquatic ecosystems
• N saturation of terrestrial ecosystems
• Climate forcing
• Stratospheric chemistry and ozone
• Smog
• Acid rain

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Nitrogen Chemistry

• Nitrogen has 5 valence electrons and N-compounds
  can take on oxidation state ranging from 5 to -3
• Because the radius of a nitrogen atom is small
  relative to other elements (0.070 nm), nitrogen
  atoms come close enough together to form very
  strong bonds.
• nitrogen atoms can gain eight valence electrons
  by sharing three pairs of electrons with another
  nitrogen atom forming N2
• This N-N triple bond is very strong with a bond
  dissociation enthalpy of 946 KJ/mol
• Lithium is one of a few elements that will react
  with N2 at low temperature
• 6 Li(s) N2(g) 2 Li3N(s)

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Nitric Acid HNO3

• Nitric acid is a very strong acid and exists in
  the atmosphere in gas, aerosol and droplet forms
• In nature, HNO3 is produced in the troposphere by
  reaction of NO (more later)
• Much of the nitric acid produced in the world is
  manufactured via a high-temperature catalytic
  oxidation of ammonia. This process consists of
  three main steps
• ammonia oxidation
• nitric oxide oxidation
• absorption
• When reacted with a base, nitrate salts are
  produced
• NH3(g) HNO3(s or g) ? NH4NO3(s or aq)
• There are numerous other nitrate salts which are
  typically have high-melting pts, are colorless
  and very soluble in water (e.g. KNO3 and NaNO3)

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Nitrogen Oxides NO2 and NO

• Nitrogen oxides, or NOx, are a group of highly
  reactive gases, all of which contain nitrogen and
  oxygen in varying amounts
• examples of nitrogen oxides include nitric oxide
  (NO) and dinitrogen trioxide (N2O3)
• Nitrogen oxides form when fuel is burned at high
  temperatures
• Primary manmade sources of NOx are
• motor vehicles
• electric utilities
• industrial, commercial, and residential
• NOx can also be formed naturally (more later
• NO2 in the air can often be seen as a
  reddish-brown layer over many urban areas due to
  its light absorbance at 680 nm
• NO2 is a regulated criteria pollutant because it
  is the most common N form in the air that is
  generated by human activities
• NO2 can irritate the lungs and lower resistance
  to respiratory infections

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Nitrous Oxide N2O

• N20, is also known as dinitrogen oxide or
  dinitrogen monoxide and
• Is a colorless non-flammable gas
• While similar in structure to CO2 it is less
  reactive and less soluble in water
• Nitrous oxide is present in the atmosphere where
  it acts as a powerful greenhouse gas and is a
  primary control on O3 in the stratosphere
  (tropospheric residence time 150 years)
• The N20 molecule is a linear chain of a nitrogen
  atoms bound to a oxygen atom. It can be
  considered a resonance hybrid of
• N-NO and N N - O-

• N20 is produced by both natural and human-related
  sources
• Agricultural soils
• Animal manure
• Sewage treatment
• Fossil fuel combustion
• Acid production
• Biological sources in soil and water,
  particularly in wet tropical forests

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Dinitrogen N2

• N2 is considered the most stable molecule of N,
  however in the presence of O2 and water it is
  thermodynamically unstable and will form NO3-
• This reaction is however very very slow given the
  high activation energy required to break the NN
  bond
• N2 has low solubility in water, yet it is the
  most abundant N species in the ocean why?
• LN2 is produced industrially by liquefaction and
  distillation of air
• Engineers are working on cars powered by LN2
• LN2 conversion from liquid to gas expands the
  volume of the nitrogen 700 times and can be used
  to drive an air motor much like burning gasoline
  drives an internal combustion engine

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Reduce Nitrogenous Compounds

• Amines
• R-NH2 are organic derivatives of ammonia where an
  alkyl group replaces one of the Hs
• The simplest amines are methylamines CH3-NH2,
  (CH3)2-NH and (CH3)3-N
• Simple, polar amines are soluble in water, basic
  and participate in hydrogen bonding
• With larger R groups, amines take on the
  properties of organic molecules and become less
  soluble in water
• Amides
• Urea NH2-CO-NH2 is an important N exchange
  between animals (excretion) and plants (uptake
  via breakdown in NH3
• Animal wastes and fertilizer are major
  anthropogenic sources of urea to the atmosphere)

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Ammonia NH3

• Ammonia is a strong base
• Ammonia comprises the bulk of reduced N in the
  atmosphere, existing primarily as aqueous or
  aerosol ammonium ion with lesser amounts of
  gaseous NH3
• At the turn of the 20th century there was a
  shortage of naturally occurring N fertilizers
  which prompted chemists to look for ways of
  combining N2 with hydrogen to form ammonia
• Germany was preparing for World War I and
  especially needed reduced nitrogen compounds for
  production of explosives (ammonium nitrate)
  this lead to the discovery of the Haber process
• Millions of metric tons of ammonia are
  manufactured each year for fertilizer production
  by the Haber-process
• N2 3H2 ? 2NH3

Fritz Haber won the Nobel Prize in 1918 for the
discovery of the reaction named after him Haber
is also remembered for contributions he made to
the studies of electrolysis of solid salts,
Bunsen flames and energy loss from engines,
motors and turbines He has been vilified for
developing the use of chlorine gas during WWI a
weapon he thought would help bring a swift
victory and thus limit overall suffering Others
took a dimmer view including his wife who,
tormented by her husbands actions, committed
suicide
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Industrial N Fixation The Process

• In 1909 Haber discovered the conditions under
  which N2 and H2 would react
• medium temperature (500C)
• high pressure (250 atmospheres, 351kPa)
• Fe catalyst
• This process produces an ammonia yield of
  approximately 10-20
• The Haber process was subsequently developed into
  an industrial process by C. Bosch
• The reaction between nitrogen gas and hydrogen
  gas to produce ammonia gas is exothermic,
  releasing 92.4kJ/mol of energy at 298K (25C)
• H2 used in the process was produced by high
  temperature decomposition of water at first and
  is now obtained by reforming light petroleum
  fractions or methane by adding steam

CH4(g) H2O(g) Ni 700C ?CO(g) 3H2(g)
2CH4(g) O2(g) 4N2(g) Ni? 2CO(g) 4H2(g)
4N2(g) CO(g) H2O(g) FeO catalyst ?H2(g)
CO2(g) CO2 is then removed using a base so that
only N2 and H2 gases remain N2 3H2 ? 2NH3
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Ramifications of Industrial N fixation
Smil, 1998 Scientific American
200 million tons of N are applied each year 50
is incorporated into cultivated plants providing
about 40 of plant N uptake Crops provide 75
of N in proteins consumed by humans Therefore
about 1/3 of protein in humanitys diet depends
on synthetic N fertilizer
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Key Biological N Transformations

• N-fixation
• NH3 assimilation
• Nitrification
• Assimilatory nitrate reduction
• Ammonification
• Dissimilatory nitrate reduction to ammonium
• Denitrification

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N-fixation

• Biological N-fixation was the ultimate source of
  N in living organisms prior to industrial
  fertilizer production and fossil fuel combustion
• What organisms fix N?
• Where does this occur?
• Why does the process take so much energy?
• How do organisms overcome this energy hurdle?

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Ammonia Assimilation

• After N-fixation the NH3 or NH4 produced can
  follow what two pathways
• 1.?
• 2.?
• 3. What forms of N do plants use?
• 4. Which form is most energy efficient?

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Nitrification

• Nitrification consist of two energy yielding
  steps what are they and which one produces more
  energy?
• ?
• ?
• Which bacterial genus is involved in each step?
• Are these autotrophic or heterotrophic organisms?

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Assimilatory Nitrate Reduction

• Nitrate can serve as a terminal electron acceptor
  under anaerobic conditions or it can be reduced
  and assimilated into an organism
• 1. When is this process most likely to occur
• 2. Why would an organism conduct this type of
  metabolism?
• Answer
• Organisms can get NH2 from pre-existing
  molecules, NH3 and NH4, (most energy-efficient
  route) or they can synthesize amino groups within
  their cells by assimilatory nitrate reduction
• This is a common scenario for plants, fungi and
  various prokaryotes since under aerobic
  conditions nitrate is often available while
  ammonia is not
• Nitrate is reduced by series of enzymes
  (dehydrogenases) to an amino group
• NO3?NO2 ? NH2OH ? NH3 ? R-NH2
• Note Dissimilatory nitrate reduction (aka
  denitrification) occurs during anaerobic
  respiration. In DSR nitrate is used as an
  external electron acceptor, which forms a variety
  of reduced products ranging from nitrite to
  dinitrogen gas. These are discarded as wastes,
  rather than assimilated into cell's organic
  molecules as they are in ANR

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Ammonification

• What is this process?
• What types of bacteria are typically involved?

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Denitrification

• What is the major significance of
  denitrification?
• Balances N-fixation and removes N from
  biosphere. Helps maintain N-limitation of
  terrestrial ecosystems
• What organisms conduct denitrification and under
  what conditions?
• Usually facultative anaerobes
• Predominantly
• Pseudomonas spp.
• Bacillus spp.
• What are the end products of this process?
• Compounds ranging from NO2 to N2
• What are there typical proportions?
• Typically N2 comprises 80-100 of the products
  of denitrification on a world-wide basis

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Routes for Anthropogenic N Fixation

• Intentional
• Production of NH3 and HNO3 fertilizers
• Planting of N-fixing crops like legumes and
  clover to replenish farmlands
• Unintentional
• Production of NO, from atmospheric N2 and O2
  during high temperature combustion of fossil
  fuels and biomass

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Atmospheric Chemistry of N

• Atmospheric exchange of N2 via biological
  fixation and denitrification dominate the
  biosphere-atmosphere cycle
• However, cycling of less abundant N species
  between the biosphere and atmosphere are still
  critically important to global biogeochemical
  cycles
• Lets review some of the most important
  homogeneous (gas-gas) and heterogeneous
  (gas-liquid-solid) reactions

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Atmospheric N Reactions 1

• Photochemically produced odd electron species
  (radicals such as OH, H-O2 and R-O2) are
  responsible for most of the oxidizing reactions
  of N species in the atmosphere
• Remember
• O3 hv ? O2 O (1D electronic state)
• O H2O ? 2 OH or
• O CH4 ? OH CH3 
• Termolecular process
• O2 O M ? O3 M
• Where M is a third body that carries away some of
  the vibrational energy generated by the
  exothermic reaction thereby stabilizing the
  products of the reaction in the atmosphere M is
  usually N2 or O2

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Atmospheric N Reactions 2

• Some tropospheric oxidation reactions worth
  remembering
• OH is the primary oxidizer in the atmosphere and
  is responsble for oxidizing
• CH4 and hydrocarbons to CH2O, CO and CO2
• CO to CO2
• NO to NO2
• NO2 to HNO3
• Example 1 coupled oxidation of methane and NO
• OH CH4 ? H2O CH3 (methyl radical)
• M CH3 O2 ? CH3O2 (methylperoxy radical )
  M
• CH3O2 NO ? NO2 CH3O (methoxy radical)
• CH3O O2 ? CH2O HO2 (hydroperoxy radical)
• HO2 NO ? NO2 OH

OH is regenerated thus it acts as a catalyst for
the reaction
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Atmospheric N Reactions 3

• Example 2 - oxidation of CO to CO2
• OH CO ? CO2 H 
• H O2 ? HO2
• HO2 NO ? OH NO2 
• Example 3 oxidation of NO2 to HNO3
• NO2 OH M ? HNO3 M
• HNO3 is then removed from atmosphere via wet or
  dry deposition as acid deposition

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Atmospheric N Reactions 4

• Example 4 coupling of NO oxidation to ozone
  formation
• NO2 hv ? NO O (hv lt 410 nm wavelength.
  NOgt 30 ppt)
• O O2 M ? O3 M
• These reactions can be reversed by the rapid
  reaction of NO with ozone
• The rate if the forward and reverse reactions are
  dependant on the concentration of NOx (NOx
  NO NO2) and light intensity
• On sunny summer days ozone production is high,
  however, continued production of NO at night can
  consume O3
• This fact often causes night-time ozone levels in
  urban areas to be lower than in downwind rural
  areas with no mobile NO sources
• Ozone negatively impacts the process of
  photosynthesis by reducing the amount of carbon
  dioxide plants are able to process

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Ozone in National Parks is Higher than in the
Cities
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Photochemical Smog And The Role of VOCs

• Example 5
• A frequent constituent of smog is the powerful
  eye irritant PAN, peroxyacetylnitrate
• It is formed from acetaldehyde, CH3CHO, a product
  of the photo-oxidation of ethane and other
  hydrocarbons plus natural emissions from trees
  (e.g., Blue Ridge Mts)
•                 CH3CHO OH H2O CH3CO
  (acetic acid radical)
•                  CH3CO O2 CH3COO2
  (peroxyacetyl radical)
•                CH3COO2 NO2 CH3COO2NO2 
  (PAN) 
• PAN can dissociate again to form NO2, thus it can
  act as a transport agent of NO2 to remote areas
  far from the pollution source (remember our
  discussion of atmospheric organic nitrogen AON?)

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Stratospheric N Reactions

• Stratospheric O3 is formed by the same reactions
  as in the troposphere, however concentrations are
  much higher due to greater O levels due to
  higher UV light
• Under natural conditions stratospheric O3
  production is balanced by radical destruction
  reactions
• O3 X ? XO O2
• XO O ? X O2
• Where X is any radical NO, H etc.
• e.g.
• O3 NO ? NO2 O2
• NO2 O ? NO O2
• The major source of stratospheric NO is the slow
  upward diffusion of N2O through the tropopause
  and its photolysis and reaction with O
• N2O hv ? N2 O
• N2O O ? 2NO
• When and where can NO actually conserve Ozone?

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Heterogeneous Atmospheric N Reactions

• Heterogeneous processes are involved in the
  atmospheric N cycle in the following ways
• Gas particle conversion
• Gas uptake by cloudwater and precipitation
• Exchange of N-gases between the ocean and
  atmosphere
• Exchange of N-gases between soils and the
  atmosphere
• In the atmosphere precipitation is the most
  important means to remove NH4 and NO3- ions
  since these N species are very soluble in water
• For ammonia and ammonium the condensed phases,
  liquid and solid, makeup about ? of the
  atmospheric burden whereas for nitrate about ? is
  in the gas phase

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For next time

• Finish Chapter 12
• Do problems 12-1, 12-3, 12-4, 12-5, 12-6, 12-8