Th role
The
l off nitrogen
it
in
i th
the
environment
nitrogen cycle
nitrogen fixation
the Haber-Bosch process
p
nitrogen pollution
The nitrogen cycle
The nitrogen cycle
The nitrogen cycle is the biogeochemical cycle that
describes the transformations of nitrogen and nitrogencontaining compounds in nature. It is a gaseous cycle. The
Earth's atmosphere is about 78% nitrogen, making it the
largest pool of nitrogen. Nitrogen is essential for many
biological processes; and is crucial for any life here on
Earth. It is in all amino acids, is incorporated into proteins,
and is present in the bases that make up nucleic acids
acids, such
as DNA and RNA.
Processing, or fixation, is necessary to convert gaseous
nitrogen into forms usable by living organisms. Some fixation
occurs
occu
s in lightning
g t g st
strikes,
es, but most
ost fixation
at o is
s do
done
e by freeee
living or symbiotic bacteria.
Nitrogen fixation
Nitrogen-fixing bacteria have the nitrogenase enzyme that
combines gaseous nitrogen with hydrogen to produce
ammonia, which is then further converted by the bacteria
to make its own organic compounds. Some nitrogen fixing
bacteria, such as Rhizobium, live in the root nodules of
legumes (such as peas or beans). Here they form a
mutualistic relationship with the plant, producing ammonia
in exchange for carbohydrates
carbohydrates. Nutrient
Nutrient-poor
poor soils can be
planted with legumes to enrich them with nitrogen. A few
other plants can form such symbioses.
There are also bacteria species such as Azotobacter that
are
a
e capab
capable
eo
of nitrogen
t oge fixation
at o in tthe
e so
soil.
Amplification of nitrogen fixation
Nitrogen fixation has been thoroughly studied in recent
years, based on the hope that genetic engineering can
provide techniques that improve the nitrogen supply of plants.
The production of synthetic nitrogen fertiliser is expensive
and extraordinarily costly in terms of energy. Bacteria, too,
are not able to produce ammonia at low energy costs.
The triple bond of nitrogen belongs to the strongest covalent
bonds occurring in biologically important molecules
molecules. The
conversion of 1 mole nitrogen to 2 mole ammonia requires
25 mole ATP, i.e. the fixation of 1 gm. nitrogen costs 10 gm.
glucose - under favourable conditions. Azotobacter’s reaction
is especially pricey: it needs 100 gm. glucose for the fixation
of 1 g
o
gm. nitrogen.
t oge
Nitrogen fixation
The genetic basis of nitrogen fixation is largely known.
The preferred test object was and still is Klebsiella
pneumoniae, an enterobacterium related to Eschericia coli.
In nitrogen fixation, the nitrogenase complex is the key enzyme.
The reduction of molecular
N2 to NH3, is catalysed by
the nitrogenase enzyme
system (EC 1
1.18.6.1).
18 6 1)
The overall reaction is:
N2 + 8 H + + 8 e -
2 NH3 + H2
This process consumes 16 ATP
o ecu es
molecules:
16 ATP
16 ADP + 16 P
Nitrogenase: active site components
Molybdenum nitrogenase (Mo nitrogenase), which is found
in all nitrogen fixing organisms, consists of two components:
component II
component I
[nitrogenase iron (Fe) protein
protein,
[nitrogenase mol
molybdenum-iron
bden m iron
(MoFe) protein, or dinitrogenase] or dinitrogenase reductase]
This is the site of N2 reduction.
Electron transfer protein.
Nitrogen fixation: a laboratory model
An electrochemical
A
l t h i l system
t
will
ill convertt N2 to
t NH3
In the laboratory. Bonds between the nitrogen atoms
Break in stages with bonds forming between the nitrogen
And molybdenum at the same time.
Nitrogen fixation: energetics
Even if the biological process does not involve tearing apart
the nitrogen molecule, but goes along in stages from N2 to
diazene (N2H2), then to hydrazine (N2H4), and finally to NH3,
there is still an energy problem. This pathway results in the
overall release of energy only when the last stage - the
production of ammonia - is reached
reached. To carry the system over
the energy barrier, a lot of energy must be added to the system.
This is why symbiotic relationships between bacteria and plants
are common in nitrogen fixation.
185 kJ/mol
N2H2
95 kJ/mol
N2H4
N2
185 kJ/mol
NH3
The fate of nitrogen in the soil
Other plants get nitrogen from the soil by absorption at their
roots in the form of either nitrate ions or ammonium ions. All
nitrogen obtained by animals can be traced back to the
eating of plants at some stage of the food chain.
Due to their very high solubility, nitrates can enter groundwater.
Elevated nitrate in groundwater is a concern for drinking water
use because nitrate can interfere with blood-oxygen levels in
infants and cause methemoglobinemia or blue
blue-baby
baby syndrome
syndrome.
Where groundwater recharges stream flow, nitrate-enriched
groundwater can contribute to eutrophication, a process
leading to high blue-green algal populations and the death
of aquatic life due to excessive demand for oxygen. While
ot directly
d ect y to
toxic
c to fish
s life
e like
ea
ammonia,
o a, nitrate
t ate ca
can have
a e
not
indirect effects on fish if it contributes to this eutrophication.
Nitrogen assimilation (E. coli)
Nitrogen limitation in Escherichia coli controls the
expression of about 100 genes of the nitrogen regulated
(Ntr) response,
response including the ammonia
ammonia-assimilating
assimilating
glutamine synthetase. Low intracellular glutamine controls
the Ntr response through several regulators.
Ntr proteins assimilate ammonia, scavenge nitrogencontaining compounds, and appear to integrate ammonia
assimilation with other aspects
p
of metabolism,, such as
polyamine metabolism and glutamate synthesis. The
leucine-responsive regulatory protein (Lrp) controls the
synthesis of glutamate synthase,
synthase which controls the Ntr
response, presumably through its effect on intracellular
glutamine.
L. Reizer Ann. Rev. Microbiol. 2003, 57, 155
Glutamine
Glutamine is the most abundant naturally occurring,
non-essential amino acid in the human body. It is found
circulating in the blood as well as stored in the
skeletal muscles. It becomes conditionally essential
(requiring intake from food or supplements) in states of
illness or injury
injury.
Glutamine has a variety of biochemical functions including:
A substrate for DNA synthesis
Major role in protein synthesis
Primary source of fuel for cells of the small intestine
Precursor for rapidly dividing immune cells
Regulation of acid-base balance in the kidney
Alternative source of fuel for the brain and helps to
block cortisol
cortisol-induced
induced protein catabolism
Nitrogen as a waste product
Nitrogen has contributed to severe eutrophication problems
in some water bodies. As of 2006, the application of nitrogen
fertilizer is being increasingly controlled in Britain and the
United States. This is occurring along the same lines as
control of phosphorus fertilizer, restriction of which is normally
considered essential to the recovery of eutrophied
waterbodies.
Ammonia is highly toxic to fish life and the water discharge
level of ammonia from wastewater treatment plants must
often be closely monitored. To prevent loss of fish, nitrification
prior to discharge is often desirable. Land application can be
an att
a
attractive
act e alternative
a te at e to tthe
e mechanical
ec a ca aeration
ae at o needed
eeded for
o
nitrification.
The Haber-Bosch process
In the Haber Process, nitrogen (N2) and hydrogen (H2)
gases are reacted over an iron catalyst
g
y ((Fe3+) in which
aluminium oxide (Al2O3) and potassium oxide (K2O) are
used as promoters. The reaction is carried out under
conditions of 250 atmospheres (atm)
(atm), 450-500 °C;
C; resulting
in a yield of 10-20%:
N2(g) + 3H2(g) → 2NH3(g) ΔHo = -92.4 kJ/mol
(Wh
(Where
ΔHo is
i th
the standard
t d dh
heatt off reaction
ti or standard
t d d
enthalpy change)
These conditions are chosen due to the high
g reaction rate
which they foster despite the poor relative amount of
ammonia produced.
History
The process was first patented by Fritz Haber. In 1910
Carl Bosch successfully commercialized the process at BASF
and secured further patents. Haber and Bosch were later
awarded Nobel prizes, in 1918 and 1931 respectively, for their
work in overcoming the chemical and engineering problems
posed by the use of large-scale high-pressure technology.
Ammonia was first manufactured using the Haber process on
an industrial scale in Germany during World War I to meet the
high demand for ammonium nitrate (for use in explosives) at a
time when supply of Chile saltpeter from Chile could not be
guaranteed because this industry was then almost 100% in
British hands. It has been suggested that without this process,
Germany
Ge
a y would
ou d a
almost
ost ce
certainly
ta y have
a e run
u out o
of e
explosives
p os es by
1916, thereby ending the war.
Synthesis gas preparation
One must obtain hydrogen from methane using heterogeneous catalysis for the Haber-Bosch process.
First, the methane is cleaned, mainly to remove sulphur
impurities that would poison the catalysts. This is done by
turning sulphur into hydrogen sulphide:
CH3SH + H2 → CH4 + H2S
and then reacting with zinc oxide to form zinc sulphide:
H2S + ZnO → ZnS + H2O
The clean methane is then reacted with steam over a
catalyst
t l t off nickel
i k l oxide.
id Thi
This iis called
ll d steam
t
reforming:
f
i
CH4 + H2O → CO + 3H2 (3 moles of hydrogen out)
CO + H2O → CO2 + H2 (1 extra mole of hydrogen out)
Note that 4 moles of hydrogen are produced per mole of
methane
Reaction Rates and Equilibrium
There are two opposing considerations in this synthesis:
the position of the equilibrium and the rate of reaction. At room
temperature, the reaction is slow and the obvious solution is
to raise the temperature. This may increase the rate of the
reaction but, since the reaction is exothermic, it also has the
effect, according to Le Chatelier's Principle, of favouring the
reverse reaction and thus reducing equilibrium constant, given
by:
As the temperature increases, the equilibrium is shifted and
hence, the constant drops dramatically according to the
a t Hoff
o equat
equation.
o Lower
o e te
temperatures
pe atu es ca
cannot
ot be used ssince
ce
van't
the catalyst itself requires a temperature of at least 400 °C
to be efficient.
Reaction Rates and Equilibrium
Pressure is the obvious choice to favour the forward reaction
because there are 4 moles of reactant for every 2 moles of
product, and the pressure used (around 200 atm) alters the
equilibrium concentrations to give a profitable yield.
Economically, though, pressure is an expensive commodity.
Pipes and reaction vessels need to be strengthened, valves
more rigorous, and there are safety considerations of working
at 200 atm.
atm In addition,
addition running pumps and compressors takes
considerable energy. Thus the compromise used gives a single
pass yield of around 15%. Another way to increase the yield
of the reaction would be to remove the product (i.e. ammonia
gas) from the system. In practice, gaseous ammonia is not
e o ed from
o tthe
e reactor
eacto itself,
tse , ssince
ce tthe
e te
temperature
pe atu e is
s too
removed
high; but it is removed from the equilibrium mixture of gases
leaving the reaction vessel.
Reactive N
vs
Unreactive N2
• Unreactive N is N2 (78% of earth’s atmosphere)
• Reactive N (Nr) includes all biologically,
chemically and physically active N compounds in
the atmosphere and biosphere of the Earth
• N controls productivity of most natural
ecosystems
• N2 is converted to Nr by biological nitrogen
fixation (BNF)
• N2 is converted to Nr by humans fossil fuel
combustion the Haber Bosch process
combustion,
process, and
cultivation-induced BNF.
Reactive N
•
•
•
•
•
•
vs
Unreactive N2
Unreactive N is N2 (78% of earth’s atmosphere)
Reactive N (Nr) includes all biologically, chemically and
physically
p
y
y active N compounds
p
in the atmosphere
p
and
biosphere of the Earth
N controls productivity of most natural ecosystems
N2 is converted to Nr by biological nitrogen fixation (BNF)
N2 is converted to Nr by humans fossil fuel combustion, the
Haber Bosch process, and cultivation-induced BNF.
Bottom Lines
– Humans create more Nr than do natural terrestrial processes.
– Nr is accumulating in the environment.
– Nr accumulation contributes to most environment issues of
the day.
day
– Challenge is to reduce anthropogenic Nr creation.
Reactive N
•
•
•
•
•
•
vs
Unreactive N2
Unreactive N is N2 (78% of earth’s atmosphere)
Reactive N (Nr) includes all biologically, chemically and
physically
p
y
y active N compounds
p
in the atmosphere
p
and
biosphere of the Earth
N controls productivity of most natural ecosystems
N2 is converted to Nr by biological nitrogen fixation (BNF)
N2 is converted to Nr by humans fossil fuel combustion, the
Haber Bosch process, and cultivation-induced BNF.
Bottom Lines
– Humans create more Nr than do natural terrestrial processes.
– Nr is accumulating in the environment.
– Nr accumulation contributes to most environment issues of
the day.
day
– Challenge is to reduce anthropogenic Nr creation.
•
But, this is complicated by fact that Nr creation sustains
most of the world
world’s
s food needs.
– The real challenge is how can we provide food (and energy)
while also reducing Nr creation rates and arresting the
nitrogen cascade?
Impact of Nitrogen
Historical perspective
– Human discovery; human ingenuity
– N cycle in 1860 and 1995
Consequences of being ingenious
– Nitrogen is nutritious
– Nitrogen
g cascades
How can one atom do all those things?
– Impacts on atmosphere
– Impacts on grasslands, forests and
agroecosystems
– Impacts on freshwater, coastal waters and
oceans
Human
n popula
ation (millions)
The History of Nitrogen
7,000
6,000
5,000
4,000
3 000
3,000
2,000
N-Discovered N-Nutrient
BNF
1,000
,
0
1750
1800
1850
1900
1950
2000
2050
Humans millions
Humans,
Year
Galloway JN and Cowling EB. 2002; Galloway et al., 200
7,000
200
6,000
5,000
150
N2 + 3H2
--> 2NH3
4,000
100
3 000
3,000
2,000
N-Discovered N-Nutrient
BNF H-B
50
1,000
,
N2 + O2
--> 2NO
0
1750
1800
1850
Humans, millions
Legumes/Rice, Tg N
1900
1950
2000
0
2050
NOx e
emissions (Tg/yyear)
Human
n popula
ation (millions)
Nr Creation by
y Haber-Bosch
Haber Bosch
NOx emissions, Tg N
Galloway JN and Cowling EB. 2002; Galloway et al., 200
The Global Nitrogen Budget in 1860 and mid-1990s, TgN/yr
186
60
NOy
5
N2
NHx
8
6
6
6
9
120
7
11 8
15
0.3
mid-1
1990s
27
NOy
5
N2
NHx
33
16
21
25
110
25
6
23 26
18
39
100
N2 + 3H2
48
2NH3
Galloway et al., 2002b
Atmosphere
Terrestrial
Ecosyste
ms
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Aquatic Ecosystems
Atmosphere
NOx
Ozone
Effects
Energy
Production
Terrestrial
Ecosystems
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Aquatic Ecosystems
Atmosphere
NOx
Ozone
Effects
Air Quality
Vi ibilit
Visibility
Effects
Energy
Production
Terrestrial
Ecosystems
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Aquatic Ecosystems
Atmosphere
NOx
Ozone
Effects
Air Quality
Vi ibilit
Visibility
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Aquatic Ecosystems
Atmosphere
NOx
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Groundwater
Effects
Surface water
Effects
Aquatic Ecosystems
Atmosphere
NOx
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Aquatic Ecosystems
Atmosphere
NOx
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Atmosphere
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Terrestrial
Ecosystems
Food
Production
NHx
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
A i l
Animal
Soil
Forests &
Grassland
Soil
Norg
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Atmosphere
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
C
Cascade
d
Galloway et al., 2002a
A i l
Animal
Soil
Norg
Terrestrial
Ecosystems
Forests &
Grassland
Soil
NO3
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Atmosphere
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
C
Cascade
d
--Indicates denitrification potential
A i l
Animal
Soil
Norg
Terrestrial
Ecosystems
Forests &
Grassland
Soil
NO3
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Stratospheric
Effects
Atmosphere
Air Quality
Vi ibilit
Visibility
Effects
Ozone
Effects
NOx
Energy
Production
Food
Production
NOx
NHx
NH3
Agroecosystem Effects
Crop
People
(Food; Fiber)
Human Activities
The Nitrogen
C
Cascade
d
--Indicates denitrification potential
A i l
Animal
Soil
Norg
Terrestrial
Ecosystems
GH
Effect
s
N 2O
Forests &
Grassland
Soil
NO3
N 2O
Groundwater
Effects
Surface water
Effects
Coastal
Effects
Ocean
Effects
Aquatic Ecosystems
Nr and Agricultural Ecosystems
•
Haber-Bosch has facilitated
agricultural intensification
•
40% of world’s population is
alive because of it
•
An additional 3 billion people
by 2050 will be sustained by it
•
Most N that enters
agroecosystems is released to
the environment.
Nr and the Atmosphere
‹ NOx emissions contribute to
OH, which defines the
oxidizing capacity of the
atmosphere
‹ NOx emissions are responsible
for tens of thousands of excessdeaths per year in the United
States
‹ O3 and N2O contribute to
atmospheric warming
‹ N2O emissions contribute to
stratospheric O3 depletion
Nr and Terrestrial Ecosystems
•
N is the limiting nutrient in
most temperate and polar
ecosystems
•
Nr deposition increases and
then decreases forest and
grassland productivity
•
Nr additions probably decrease
biodiversity across the entire
range of deposition
Nr and Freshwater Ecosystems
• Surface water acidification
– Tens of thousands of lakes
and streams
– Significant biodiversity
losses
– Negative
N ti feedbacks
f db k tto
forested ecosystems
Nr and Coastal Ecosystems
•
•
•
Riverine and atmospheric
deposition are significant Nr
sources to coastal systems
Nr inputs into coastal regions
result in eutrophication,
biodiversity losses, emissions
of N2O to the atmosphere
atmosphere.
Most coastal regions are
impacted.
There are significant effects
of Nr accumulation within each
reservoir
These effects are linked temporally
and biogeochemically in the
Nitrogen Cascade
Nr Riverine Fluxes
1860 (left) and 1990 (right)
TgN/yr
9.1
4.4
21.8
5
7.8
8.3
85
7.7 8.5
7.4
9.7
2
-> all regions increase riverine fluxes
-> Asia
A i becomes
b
dominant
d i
t
Galloway et al, 2002b; Boyer et al., in preparation
2.1
Nitrogen Deposition
Past and Present
mg N/m2/yr
5000
2000
1000
750
500
250
100
50
25
5
1860
1993
Galloway and Cowling, 2002; Galloway et al., 2002