CHEM3061_Unit2B_Notes.pdf

B. Nitrogen Cycling and Chemistry
Environmental Forms of N
87
Dr. Cora Young, CHEM 3061, 2024
• Numerous forms of N commonly found in the environment.
– Range from oxidation states of N from -3 to 5.
N Species
N Oxidation
State
HNO3, NO3
-
V
NO2
IV
HNO2, NO2
-
III
NO
II
N2O
I
N2
0
NH3, NH4
+
-III
Organic N
Varied
• Generally classified into total N
and reactive N.
• Total N (Ntot) is the sum of all N-
containing species.
• Reactive N (Nr) is the sum of all N-
containing species that are
considered reactive.

88
• Some forms of N have acid and base properties.
– Oxidized forms can be acidic.
– Reduced forms can be basic.
Location
Properties
Reactive
N?
N Species
N Oxidation
State
Condensed
Strong acid, pKa = -1.5
Yes
HNO3, NO3
-
V
Gas
Yes
NO2
IV
Gas/Condensed
Weak acid, pKa = 3.25
Yes
HNO2, NO2
-
III
Gas
Yes
NO
II
Gas
N2O
I
Gas
N2
0
Gas/Condensed
Weak base, pKa = 9.24
Yes
NH3, NH4
+
-III
Gas/Condensed
Yes
Organic N
Varied

A. NO
B. HNO2
C. N2O
D. NO2
E. NO3
-
Dr. Cora Young, CHEM 3061, 2024 89
Which form of N is not considered part of reactive N?

Natural Transformations
90
• N is an essential element to life on Earth.
• The largest reservoir of nitrogen on earth is atmospheric N2
(3.9 x 106 Pg N).
• N2 is highly inert and cannot be used by most lifeforms.
• All N that is available for use in the biosphere was originally
derived from nitrogen fixation.
Bond energy
(kJ/mol)
Bond
946
N≡N
389
N-H
293
N-C
201
N-O
– Nitrogen fixation is the process
of converting N2 to other forms
of N.
– Occurs through lightning.
– A few specialized organisms.

91
1. Fixation through lightning
N2 + O2 → 2NO
N2 + 8H+ + 8e- + 16ATP + 16H2O → 2NH3 + H2 + 16ADP + 16PO4
3-
2. Biological fixation
– Production of Nr 9 Tg N/yr.
– Reaction catalyzed by Mo-containing enzyme, nitrogenase.
– A few specialized organisms.
– Very energy intensive, thought to account for ~2.5 % of
global net primary productivity.
– Production of Nr on land 60-195 Tg N/yr.

92
• Nr from lightning is formed as NO in the atmosphere.
– It can subsequently undergo several transformations.
NO NO2
HNO3
This Photo by Unknown Author is licensed under CC BY-NC
O3 O2
hν
O
OrgN
OrgN
OH

93
• Nr from biological fixation is formed as NH4
+ in the biosphere.
– It can subsequently undergo additional biological
transformations.
NO2
-
Nitrification
NO3
- N2
Denitrification
OrgN Photosynthesis
Decay
NO N2O
NO2
-
NH4
+

94
• Nitrification is the biological oxidation of NH4
+ to NO3
-.
– Primarily Nitrosomonas and Nitrobacter
chemoautotrophic bacteria.
– Produces NO2
- as an intermediate.
• Denitrification is the reduction of NO3
- to N2.
– Denitrifying microorganisms.
– Produces NO2
-, NO, and N2O as intermediates.
• Typical soil and aqueous systems contain a mixture of NH4
+,
NO2
-, NO3
- created through these processes.

A. i, ii
B. ii, iii
C. iii, iv
D. i, iii
E. ii, iv
Which processes lead to the production of NO?
i. N2 fixation by lightning
ii. N2 biological fixation
iii. Denitrification
iv. Photosynthesis

Speciation in Natural Waters
96
• Natural waters encompass many different conditions.
– Variety of levels of oxidative capacity.
– Variety of pHs.
• Consider the different conditions that might be present in
groundwater isolated from the atmosphere vs surface
waters.
This Photo by Unknown Author is licensed under CC BY
• Abiotic redox reactions
can also play a role in
determining which
species of N are
present.

97
Example 14. Draw a pE-pH sketch for inorganic N species
(including HNO3, HNO2, N2, and NH3) using oxidation states and
pKa values.
pH

98
• To explore this more quantitatively, we examine the redox
reactions that describe conversion between each
successively more reduced species:
N(V) to N(III), NO3
- to NO2
-:
Pankow Aquatic Chemistry Concepts 2020
½NO3
- + H+ + 2e- ½NO2
- + ½H2O pE0 = 14.15
N(III) to N(0), NO2
- to N2:
NO2
- + 4/3H+ + e- N2 + H2O pE0 = 25.13
N(0) to N(-III), NO2
- to N2:
N2 + H+ + e- NH3 pE0 = 2.11

99
Example 15.
Use pE equations to write an expression for the boundary line
between N(III) and N(V) as a function of pH.
½NO3
- + H+ + 2e- ½NO2
- + ½H2O pE0 = 14.15

100
Example 15 (con’t).

101
• Repeating this approach
for each redox pair, we
can create expressions to
describe equal
concentration conditions.
• We can then plot these
expressions to create a
pE-pH graph for N
speciation.
• But something is wrong
here…
pE

102
• N(III) cannot predominate anywhere in the pE-pH space.
• We have to remove N(III) from the diagram.
N(V) to N(0), NO3
- to N2:
NO3
- + 6/5H+ + e-
1/10N2 + H2O pE0 = 20.74
• Final pE-pH diagram for N in
natural waters shown here.
• N(III) can only be found in trace
quantities in the environment!
pE

103
Example 16.
Calculate the formal concentration of nitrous acid (FHNO2) of a
water body with a formal concentration of nitric acid (FHNO3) of
3.00 mM at pH 4.5 and pE 12.2.
Ka(HNO2) = 7.1 x 10-4
½NO3
- + H+ + 2e- ½NO2
- + ½H2O pE0 = 14.15

104
Example 16 (con’t).

105
Example 17.
On the pE-pH diagram shown
here, complete the labelling
by entering in the expected
speciation for each section.
Include any acid-base
chemistry.
24
20
16
12
8
4
0
-4
-8
-12
pE
14
12
10
8
6
4
2
pH

A. N2(g)
B. NH3
C. NO3
-
D. NH4
+
Using the pE-pH diagram we just developed, what is
the expected form of N in the surface ocean (pH = 8.1,
pe = 14.0)?

Changing N Cycle
107
• Humans have dramatically increased the rate of N fixation.
– Starting in the mid-1970s, human N fixation exceeded
natural N fixation.
• Excess Nr can contribute to numerous environmental issues.
IPCC Climate Change 2013 The Scientific Basis
• Three major processes
have led to this increased
Nr.
1. Haber-Bosch process
2. Anthropogenic
biological N fixation
3. Fossil fuel burning

Anthropogenic N Fixation
108
1. The Haber-Bosch process for synthesizing NH3 from N2
and H2 dramatically changed the Nr landscape.
– The NH3 produced from this reaction is essential for
fertilizers that feed much of humanity.
– The catalyzed
reaction operates
at elevated
temperatures and
pressures.
– Consumes 1 % of
the world’s energy.
Schlesinger and Bernhardt Biogeochemistry 2020

Anthropogenic N Fixation
109
2. Anthropogenic biological N fixation.
– Created by the increased cultivation of N-fixing plants
(e.g. legumes).
3. Fixation of N by combustion engines.
– A small amount of fixed N is derived from the N content
of fuels.
• Generally considered an Nr input, since N in fuels
would not have been accessible to the Nr cycle.
– NOx also forms from breaking N2 under the temperature
and pressure conditions of combustion.

Overall N Fixation
110
N fixation on land (Tg N/yr)
Inputs Total
Anthropogenic
Pre-industrial
120
60
60
Biological N fixation
3
0
3
Lightning
15
0
15
Rock weathering
150
150
0
Haber Bosch
25
25
0
Combustion
313
235
78
Total
Schlesinger and Bernhardt Biogeochemistry 2020
• Total production of Nr is in a regime never before observed
in Earth’s history.

Effects of Increased Atmospheric Nr
111
• Increased Nr emitted to the atmosphere contributes to:
– The formation of photochemical smog.
– Acid N deposition.
NO NO2
HNO3
This Photo by Unknown Author is licensed under CC BY-NC
RO2 RO
hν
O
OrgN
OrgN
OH
O2
O3

112
• Atmospheric deposition of oxidized Nr leads to acid rain.
• Acid rain can acidify soils.
• Particularly problematic for
soils that have a low
capability to buffer the
addition of acids.
• Acid rain was a big problem
in the second half of the 20th
century.
Jacob introduction to Atmospheric Chemistry 1999

113
• Because of the negative
impacts of acid rain, major
efforts made to reduce the
release of Nr from
combustion.
• Although emissions of Nr
per distance driven have
decreased, offset by
increased vehicle usage.
• Many of the worst effects of
acid rain were mitigated.
http://www.sudburysoilsstudy.com/

114
• We might initially think that the impacts of acids on soils
would be limited to deposition of acids.
• However, ammonium can be oxidized microbially to nitrate,
creating further acid:
NH4
+
(aq) + 2O2 + H2O → NO3
-
(aq) + 2H3O+
(aq)
• The acids in precipitation does not give the full story in
acidifying potential.
• Increased use of NH3 fertilizers continues to pose a problem
for acid-sensitive soils.

115
Example 18.
How can ammonia (produced through the Haber-Bosch
process) applied directly to farmer’s fields lead to acid soil
problems in areas far from agricultural areas?

116
• Nr from the Haber-Bosch process and biological fixation
enters the environment as NH3 in the biosphere.
– It is readily moved and transformed.
• Excessive Nr in an ecosystem can lead to eutrophication.
– One of the most serious environmental threats to
aqueous systems.
• Eutrophication is the enrichment of water by nutrients
causing an accelerated growth of algae and higher forms of
plant life that produces and undesirable disturbance to the
balance of organisms present in the water.
• Several possible undesirable changes including hypoxic
regions.

Effects of Increased Biosphere Nr
• Hypoxic conditions can occur in areas of eutrophication.
– Greatly reduced concentrations of O2.
• Creation of
hypoxic waters
requires that
biological uptake
rate of O2 exceed
resupply.
• Main O2 sources
are photosynthesis
and exchange with
the atmosphere.

• Hypoxic areas found all over the world.
• Incidences of hypoxia are increasing.
• One of the largest in the Gulf of Mexico,
where hypoxic area tripled since 1980s.
• Attributed to
agricultural runoff
from the
Mississippi River
(3rd largest
watershed; drains
parts of 31 states
and 2 provinces).

• N2O produced from
denitrification reactions are
increased as a result of
higher biosphere Nr.
• It has increased by 20 %
since pre-industrial time.
Schlesinger and Bernhardt Biogeochemistry 2020, IPCC Climate Change 2013 The Scientific Basis
• N2O is a potent long-
lived greenhouse
gas.
– ~220 times the per
mol impact of CO2.

A. Smog formation
B. Eutrophication
C. Production of N2O
D. All of the above
Effects of Increased Nr
Reducing NOx emissions from combustion sources
improved soil acidity problems. What other
environmental issue would it also be expected to
improve?

• Because Nr species can easily interconvert, their presence
in the environment is referred to as the nitrogen cascade.
• A single atom of N
can have many
sequential
environmental
effects after being
converted from inert
N to Nr.
Galloway et al. Bioscience 2003

Concept of Planetary Boundaries
• In 2009, Rockström et al. introduced the concept of a
planetary boundary.
• They propose that within a planetary boundary, humanity
(and the Earthy system) can safely exist.
Rockström et al., 2009
• Outside the planetary
boundaries, there is
increased potential for non-
linear, abrupt, and/or coupled
interactions that could lead to
negative outcomes.

Concept of Planetary Boundaries
• They defined 9 boundaries: biodiversity loss, change in land
use, biogeochemical flow, global freshwater use,
stratospheric ozone depletion, ocean acidification, climate
change, chemical pollution , and atmospheric aerosol
loading.
• Out of the 7 boundaries
they could quantify, 3 had
been surpassed in 2009.
– One was the N cycle
(part of
biogeochemical flow).

N Cycle Planetary Boundary
• Emissions of N2O are considered in the climate change
boundary.
• The N cycle boundary is set by considering a threshold on
the total quantity of N fixed.
– They estimated the boundary at 35 Mt N yr-1.
– Current fixation is about 4x that at 121 Mt N yr-1.
• The P cycle is also considered in the biogeochemical flow
boundary. Why have we not focused much on P in this
class?

125
Discussion.
As human population continues to grow, so will our need for Nr.
How can excess Nr be mitigated?

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