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The Important Nutrient Nitrogen
Nitrogen forms part of the molecules that
make up living things, such as amino acids (the
building blocks of proteins) and DNA. The nitrogen in proteins bonds together various amino
acids to form the protein structure. The amount
of nitrogen in the atmosphere is very large compared to that in the oceans or rocks. Of the elements C, N, P, S, and O, only nitrogen is found in
more abundance in the atmosphere than in rocks.
The complete biogeochemical cycle of nitro-
gen is very complex. Figures 12–17 show only
portions of it. There are six major forms of atmospheric nitrogen: the gaseous forms of diatomic
nitrogen (N2), ammonia (NH3), nitrous oxide
(N2O), and NOx (NO and NO2), and the aerosols
of ammonium (NH4+) and nitrate (NO3-). In this
chapter, we will focus on the cycles of the first
four of these forms, and also discuss nonmethane
hydrocarbons, the cycles of which are closely
related to those of NOx.
Figure 12. Part of the modern global biogeochemical cycle of nitrogen, emphasizing interactions among the land, atmosphere, and ocean.
Fluxes between the ocean, land, and groundwater are shown as arrows, with quantities given in Mt N/yr. Fluxes within reservoirs are shown
as circling arrows. “Ind. fix” is industrially fixed N (for the manufacture of fertilizers), “Bio. fix” is biologically fixed N, DN is dissolved N, PON is
particulate organic nitrogen, and “pollutant” is the excess nitrogen that has resulted from human activities (modified from Mackenzie, 1995).
at
ion
NITROGEN
(fluxes = Mt N/y)
2 fi
x
Atmospheric
CO2
N
N2O
N2O
Denitrification
1.4 – 2.6
Evasion
Land
42 Rice cultivation
20 Combustion
78 Ind. fix.
126 Bio. fix.
Enhanced organic
production-burial
216 Mt C/y
Aerosol
14
560
Ocean
Human waste
20
Agriculture
9
River
35 DN
27 PON
>21 “pollutant”
8000
Groundwater
Organic N
28
Accumulation
29
Understanding Global Change: Earth Science and Human Impacts
and rice paddy cultivation add fixed nitrogen to
the earth’s surface. Because of these human activities, the amount of nitrogen on land is increasing
(Figure 12). About 30 million tons of nitrogen are
leached from agricutural fertilizers and human
waste each year and added to groundwater systems and runoff. Some of this nitrogen makes its
way to rivers and then to lakes and the coastal
oceans. On a global scale, rivers may already
carry more nitrogen from human activities than
was transported in the natural state (Figure 12).
This increased nitrogen flux to lakes, rivers, and
coastal marine environments is one cause of
increased regional and global eutrophication of
these systems. Note, however, that rivers supply
only a small percentage of nitrogen to the coastal
zone (Figure 13). Most of the nitrogen there, other
than that recycled in the zone, upwelled from the
deep ocean to the surface.
Scientists have calculated how much this
human-caused increase in nitrogen is likely to be
N2
The overwhelming majority of nitrogen in
the atmosphere is in the form of N2. The other
forms exist only in small quantities. Biological
fixation and denitrification are the major processes leading to exchange of nitrogen between the
earth’s surface and the atmosphere (Figure 12).
Biological fixation is the process whereby N2 is
withdrawn from the atmosphere and converted
to N compounds that plants can use (e.g., NH3
and subsequently NO3-). Denitrification is the
process by which nitrogen as N2 or as N2O is
returned to the atmosphere. Both processes are
mediated by a variety of bacteria living in soils
and water.
The exchange fluxes between the earth’s surface and the atmosphere are small compared to
the internal recycling of nitrogen within the land
and ocean realms (Figures 12 and 13). Combustion
practices, the production of commercial fertilizers,
Figure 13. River input of N to the ocean compared to the fluxes involved with the internal recycling of N in the ocean due to biological productivity and decay. Besides the ocean fluxes shown, about 90% of the nitrogen involved in biological production is simply recycled in the
shallow surface waters of the coastal and open oceans. Some nitrogen escapes from the surface ocean in organic matter that settles to the
deep ocean, where the organic matter is decayed and the nitrogen released. It then returns to the surface via upwelling in the coastal zone
and vertical mixing in the open ocean. Some nitrogen, about 30 Mt per year, is buried in marine sediments in organic matter (see Figure
12). Some N is transported to the open ocean from the coastal environment (after Mackenzie, 1995; Houghton et al., 1996).
ORGANIC NITROGEN
(fluxes = Mt N/y)
Rivers
Dissolved N 35
Particulate N 27
Coastal
zone
Export
200
Surface
open ocean
Organic
Vertical
Upw
matter
e
206lling mixing sedimentation
670
900
Deep ocean
30
Global Biogeochemical Cycles and the Physical Climate System
changing ocean productivity and the flux of
organic carbon from the ocean’s euphotic zone.
The calculations show that in the 1980s there may
have been an increased organic carbon flux from
the atmosphere to the oceanic environment of
about 200 million tons of carbon per year (Figure
12), which is buried in marine sediments. This
flux takes from the atmosphere about 3% of the
increase occurring today as a result of fossil fuel
burning. While relatively small, this is a possible
negative biotic feedback on atmospheric CO2 and
hence global climate change.
N2O
N2O is a natural product of biological denitrification in soils and in the ocean (Figure 14).
The N2O produced by denitrification is only
about 15% of all N returned to the atmosphere;
the rest is in the form of N2.
N2O is an important greenhouse gas, accounting for about 9% of the enhanced greenhouse
effect since the 18th century. It has a present
atmospheric concentration of 312 ppmv and a residence time of about 130 years. This concentration
is about 8% greater than in preindustrial time and
Figure 14. Part of the modern global biogeochemical cycle of nitrous oxide. Symbols and units are as in Figure 9. This gas is responsible for
5% of the enhanced greenhouse effect. A doubling of its atmospheric concentration could lead to about a 0.4°C increase in global temperature. Notice the reaction of this long-lifetime gas in the stratosphere, leading to the destruction of stratospheric ozone. The fluxes in this
cycle are not well known (modified from Mackenzie, 1995).
NITROUS OXIDE
(fluxes = Mt N/y)
~ 85%
+ hν
<15%
+ O3
NOx
To stratosphere
5–9
N2
Climate sensitivity
0.4°C/N2O doubling
5% greenhouse
Fertiliz
er
0.01 –
2.2
Biom
ass
0.02 burnin
g
– 0.
3
Com
bu
0.1 stion
–0
.3
ean
Oc – 3
1.5
So
il
~7 s
N 2O
1510 Mt N
312 ppbv
Residence time = 130 y
Accumulation:
3.6 Mt C/y
0.8 ppbv
Land
Ocean
31
Understanding Global Change: Earth Science and Human Impacts
sediments, and ocean water. Also, N2O fluxes
from nitrogen-bearing fertilizers applied to the
land surface and sewage discharges into aquatic
systems will be affected by warming. Because the
reactions involving N2O are bacterially mediated,
it is likely that an increase in temperature will
lead to enhanced evasion rates of N2O from the
earth’s surface. This is a positive biotic feedback
on accumulation of N2O in the atmosphere and,
hence, on global warming. It could also lead to a
small enhanced destruction of stratospheric
ozone (Figure 14).
is increasing at a rate of 0.2–0.3% per year because
of human activities, including the combustion of
fossil fuels, burning of biomass, and emissions
from urea and ammonium nitrate applied to croplands. These emissions amount to 0.13 to 2.8 million tons of nitrogen annually (Figure 14).
N2O is chemically inert in the troposphere. In
the stratosphere, it can be converted photochemically to nitric oxide (NO), which acts as a catalyst
in the destruction of stratospheric ozone (see
sidebar). The series of reactions by which this is
accomplished has been one of the regulators of
stratospheric ozone concentration through geologic time.
The flux of N2O from the earth to the atmosphere has been increasing because of the rapidly
increasing use of industrially fixed nitrogen (up
to the late 1980s), increases in fossil fuel burning
and biomass burning, and increases in organic
carbon in coastal waters. This last process is an
important link between the carbon and nitrogen
cycles. The rate of denitrification and consequently of N2O emissions from coastal waters
may have increased because rivers are bringing
more organic carbon to these systems or because
these systems are undergoing eutrophication as
they receive increased inputs of nutrients from
fertilizer, sewage, and the atmosphere.
With warming, the most important biotic
feedbacks involving N2O are changes in the
denitrification (and nitrification) rates in soils,
NH3
The biogeochemical cycle of ammonia (NH3)
is shown in Figure 15. Ammonia is released to the
atmosphere by organic decomposition and
volatilization. There, it reacts with water droplets to
form ammonium ion (NH4+) and hydroxyl ion
(OH-). NH4+ appears to be removed from the
atmosphere mainly by being deposited back on
the earth in the aerosols of ammonium sulfate
[(NH4)2SO4] and ammonium nitrate (NH4NO3).
Incidentally, (NH4)2SO4 links the nitrogen and sulfur biogeochemical cycles, since its deposition on
the earth is also one of the ways oxidized sulfur is
removed from the atmosphere; the other is by
deposition of sulfuric acid (H2SO4).
Two interactions in the NH3 cycle are important in considerations of global warming. The
first is its interaction with OH* to produce NOx.
In a warmer world, the decomposition that
releases NH3 would probably be enhanced,
which would slightly increase the stress on the
OH* concentration of the atmosphere and
enhance production of NOx (Figure 16). The
effects of increased NOx concentrations are discussed in the following section. The second important interaction is NH3’s reaction with NO3 and
SO4 to produce aerosols containing ammonium
(Figure 15). Aerosols are known to cool the planet,
although the amount of the effect is unclear. An
increase in atmospheric NH3 could lead to a small
negative feedback on potential warming.
The ammonia cycle also gives us information
on nitrogen fertilization of the terrestrial biosphere. About four-fifths of the N released to the
atmosphere each year in NH3 comes from human
N2O reactions leading to the destruction
of stratospheric ozone
NO formation in the middle stratosphere (20–30 km):
N2O + ultraviolet light ⇒ NO + N
N2O + O ⇒ 2NO
(10)
(11)
Ozone destruction:
NO + O3 ⇒ NO2 + O2
O3 + ultraviolet light ⇒ O2 + O
NO2 + O ⇒ NO + O2
_____________________________________
2O3 + ultraviolet light ⇒ 3O2 (net reaction) (12)
32
Global Biogeochemical Cycles and the Physical Climate System
activities—50 out of 62 million tons. Only about
12 million tons of ammonia nitrogen per year
comes from natural bacterial decomposition in
soils. About 25% of the human-produced flux is
transported away from the continents to the
oceanic atmosphere. The rest, about 37 million
tons of nitrogen per year, falls back on the land
surface and may be available for terrestrial
organic productivity.
Now it’s time for a back-of-the-envelope calculation. If this 37 million tons of nitrogen were
to fertilize land plant production with a ratio of C
to N of 100 to 1, the plants would require more
than 3 billion tons of carbon per year. The phosphorus accumulating on land each year from
agricultural fertilizers and sewage amounts to
about 8.5 million tons (see Figure 18, p. 36)—just
about the amount of phosphorus needed to
sustain this magnitude of land plant production.
This calculation gives some idea of the potential
of fertilization of the land as a sink for the excess
CO2 that we are emitting to the atmosphere by
fossil fuel burning and land-use practices.
NOx and NMHCs
This brings us to the cycles of NOx and the
NMHCs (Figures 16 and 17). We will begin with
NOx. It has several natural sources: on the earth,
bacterial decomposition of organic matter in
soils; in the atmosphere, lightning, mixing from
the stratosphere, and oxidation of ammonia. NOx
also has anthropogenic sources: fossil fuel and
biomass burning. The main sink of NOx is
deposition on earth of chemical products that were
produced in the atmosphere by photochemical
Figure 15. Part of the modern global biogeochemical cycle of ammonia, including that of the ammonium ion (NH4+). See Figure 9 for an
explanation of the units and symbols used. Most of the ammonia emissions from the land surface are due to human activities. Ammonia is
removed from the atmosphere mainly in rain and as small, solid aerosol particles after reaction with water and with nitrate and sulfate.
Through reaction with OH*, a small amount of NH3 is converted to nitrogen oxides, e.g., NO and NO2 (modified from Mackenzie, 1995).
AMMONIA
(fluxes = Mt N/y)
epletion
OH* d
6
NO 3
Wet-dry
ion
Comb
ustio
n
7
+ H2O
89
deposit
89
Land
+
NH4
2 Mt N
Variable conc.
Residence time =
0.01 y
tion
mposi
deco
a ni c
ion
Org olatilizat
v
33
Organic
deco
mp
osi
volat
ti o
iz a t i
n
o
n
55
NH3
2 Mt N
Variable conc.
Residence time =
0.01 y
NOx
Ocean
Land
33
SO 4
Ocean
Understanding Global Change: Earth Science and Human Impacts
reactions with NOx, such as HNO3 and organic
nitrates.
NMHCs (also called volatile organic compounds, or VOCs) are natural byproducts of
plant productivity in terrestrial and marine environments. Thus, their fluxes to the atmosphere
change greatly with the seasons. They also have
anthropogenic sources—once again, fossil fuel
and biomass burning. Their main sink is in the
atmosphere, through oxidation with OH*.
effect of increasing temperature is not at all
straightforward. The concentration of ozone in
the troposphere depends in a complex way on
the atmospheric concentrations of several other
biogenic trace gases, including CH4, CO, and the
NMHCs.
In general, when there is little NOx in the troposphere (5–30 pptv), increases in the concentrations of CH4, CO, and NMHCs lead to a decrease
in the concentration of O3. At high NOx concentrations (generally greater than about 90 pptv),
increases in these three gases lead to an increase
in ozone. The combination of high NOx and
NMHCs in the troposphere disrupts the natural
cycle of production and destruction of ozone,
and ozone accumulates. In urban areas, this contributes to air pollution.
Effects of NOx on ozone
Increasing temperature alone would probably increase the flux of NOx from soils to the
atmosphere, potentially depleting OH* and forming more methane and ozone in the troposphere
(Figure 16). For tropospheric ozone, however, the
nin
g
NITROGEN OXIDES
(fluxes = Mt N/y)
5
ht
Tropospheric
O3
on
OH
*d
ep
le t
6
i
ere
Lig
NH3
F ro m
stra
tos
1 ph
Figure 16. Part of the modern biogeochemical cycle of the nitrogen oxides. About one-half of the emissions of these gases to the atmosphere comes from the combustion of fossil fuels. In the atmosphere, the gases are converted to several other chemical species, mainly
HNO3 , and removed from the atmosphere both in rain and in dust. Notice the tie to tropospheric O3 (see text), a greenhouse gas and a
component of smog (modified from Mackenzie, 1995).
Climate sensitivity
Greenhouse gas
Biomass b
urnin
g
3
ustio
n
21
Comb
Photochemical
56
HNO3
PAN
Organic nitrates
Photolysis
Ther
mal decomposition
Wet-dry
ion
deposit
56
Soil
s
20
NOx
Land
Land
34
Ocean
Global Biogeochemical Cycles and the Physical Climate System
On the other hand, increases in CH4, CO, and
NMHCs will lead to lower levels of OH*.
One critical positive feedback is that increases in CO concentrations in the atmosphere could
lead to a reduction in OH*, because NOx has too
short a lifetime to counteract that effect on a global
scale. Decreased concentrations of OH* could lead
to an increase in the lifetime of CH4, a positive,
but small, feedback on the accumulation of CH4 in
the atmosphere and hence global warming.
Effects of NOx on OH*
The concentration of OH*, which is mainly
responsible for cleansing the atmosphere,
depends on the concentrations of trace gases, tropospheric ozone, and water vapor. Elevated concentrations of O3, NOx, and H2O will increase
OH* levels. (Generally, changes in NOx concentrations affect OH* in the same way they affect
ozone, described above, except to a lesser degree.)
Figure 17. Part of the modern global biogeochemical cycle of the nonmethane hydrocarbons. Land vegetation and phytoplankton naturally
produce these compounds. Their human sources include industrial practices, transportation, and fossil fuel combustion. These compounds
react in the atmosphere with OH* and are important in controlling that compound’s concentration in the troposphere. They are also responsible for disrupting the natural production and destruction of the ozone cycle in the troposphere. In conjunction with NOx , they can lead to
increased concentrations of O3 in the troposphere (modified from Mackenzie, 1995; Guenther et al., 1995).
OH
*
NONMETHANE
HYDROCARBONS
(fluxes = Mt C/y)
tion
p le
e
d 00
13
+ NOx
Climate sensitivity
Greenhouse gas
NMHCs
n
Terr
est
r ia
l
terp
ene vege
,
t
11 isop a
45 r
tio
Tropospheric
O3
e
en
Bioma
ss bu
rnin
combu
g
stion
, sol
150 vents
an
Oce
pene
, pro
ene 5
eth
Land
Ocean
35