This passage is excerpted from the Ecological Society of America, “Human Alteration of the Global Nitrogen Cycle: Causes and Consequences,” ©1997 by the Ecological Society of America.
Nitrogen is an essential component of proteins, genetic
material, chlorophyll, and other key organic molecules. All
organisms require nitrogen in order to live. It ranks behind
oxygen, carbon, and hydrogen as the most common chemical
element in living tissues. Until human activities began to alter
the natural cycle, however, nitrogen was only scantily available
to much of the biological world. As a result, nitrogen served as
one of the major limiting factors that controlled the dynamics,
biodiversity, and functioning of many ecosystems.
The Earth’s atmosphere is 78% nitrogen gas, but most plants
and animals cannot use nitrogen gas directly from the air as
they do carbon dioxide and oxygen. Instead, plants—and all
organisms from the grazing animals to the predators to the
decomposers that ultimately secure their nourishment from the
organic materials synthesized by plants—must wait for nitrogen
to be “fixed,” that is, pulled from the air and bonded to
hydrogen or oxygen to form inorganic compounds, mainly
ammonium and nitrate, that they can use.
The amount of gaseous nitrogen being fixed at any given
time by natural processes represents only a small addition to
the pool of previously fixed nitrogen that cycles among the
living and nonliving components of the Earth’s ecosystems.
Most of that nitrogen, too, is unavailable, locked up in soil
organic matter—partially rotted plant and animal remains—that
must be decomposed by soil microbes. These microbes release
nitrogen as ammonium or nitrate, allowing it to be recycled
through the food web. The two major natural sources of new
nitrogen entering this cycle are nitrogen-fixing organisms and
lightning.
Nitrogen-fixing organisms include a relatively small number
of algae and bacteria. Many of them live free in the soil, but the
most important ones are bacteria that form close symbiotic
relationships with higher plants. Symbiotic nitrogen-fixing
bacteria such as the Rhizobia, for instance, live and work in
nodules on the roots of peas, beans, alfalfa, and other legumes.
These bacteria manufacture an enzyme that enables them to
convert gaseous nitrogen directly into plant usable forms.
Lightning may also indirectly transform atmospheric
nitrogen into nitrates, which rain onto soil.
Quantifying the rate of natural nitrogen fixation prior to
human alterations of the cycle is difficult but necessary for
evaluating the impacts of human-driven changes to the global
cycling of nitrogen. The standard unit of measurement for
analyzing the global nitrogen cycle is the teragram (abbreviated
Tg), which is equal to a million metric tons of nitrogen.
Worldwide, lighting, for instance, fixes less than 10 Tg of
nitrogen per year—maybe even less than 5 Tg. Microbes are
the major natural suppliers of new biologically available
nitrogen. Before the widespread planting of legume crops,
terrestrial organisms probably fixed between 90 and 140 Tg of
nitrogen per year. A reasonable upper bound for the rate of
natural nitrogen fixation on land is thus about 140 Tg of
nitrogen per year.
During the past century, human activities clearly have
accelerated the rate of nitrogen fixation on land, effectively
doubling the annual transfer of nitrogen from the vast but
unavailable atmospheric pool to the biologically available
forms. The major sources of this enhanced supply include
industrial processes that produce nitrogen fertilizers, the
combustion of fossil fuels, and the cultivation of soybeans,
peas, and other crops that host symbiotic nitrogen-fixing
bacteria. Furthermore, human activity is also speeding up the
release of nitrogen from long-term storage in soils and organic
matter.
Industrial fixation of nitrogen for use as fertilizer currently
totals approximately 80 Tg per year and represents by far the
largest human contribution of new nitrogen to the global cycle.
That figure does not include manures and other organic
nitrogen fertilizers, which represent a transfer of already-fixed
nitrogen from one place to another rather than new fixation.
...
Until the late 1970s, most industrially produced fertilizer
was applied in developed countries. Use in these regions has
now stabilized while fertilizer applications in developing
countries have risen more dramatically. The momentum of
human population growth and increasing urbanization ensures
that industrial fertilizer production will continue at high and
likely accelerating rates for decades in order to meet the
escalating demand for food.
Nitrogen is an essential component of proteins, genetic
material, chlorophyll, and other key organic molecules. All
organisms require nitrogen in order to live. It ranks behind
oxygen, carbon, and hydrogen as the most common chemical
element in living tissues. Until human activities began to alter
the natural cycle, however, nitrogen was only scantily available
to much of the biological world. As a result, nitrogen served as
one of the major limiting factors that controlled the dynamics,
biodiversity, and functioning of many ecosystems.
The Earth’s atmosphere is 78% nitrogen gas, but most plants
and animals cannot use nitrogen gas directly from the air as
they do carbon dioxide and oxygen. Instead, plants—and all
organisms from the grazing animals to the predators to the
decomposers that ultimately secure their nourishment from the
organic materials synthesized by plants—must wait for nitrogen
to be “fixed,” that is, pulled from the air and bonded to
hydrogen or oxygen to form inorganic compounds, mainly
ammonium and nitrate, that they can use.
The amount of gaseous nitrogen being fixed at any given
time by natural processes represents only a small addition to
the pool of previously fixed nitrogen that cycles among the
living and nonliving components of the Earth’s ecosystems.
Most of that nitrogen, too, is unavailable, locked up in soil
organic matter—partially rotted plant and animal remains—that
must be decomposed by soil microbes. These microbes release
nitrogen as ammonium or nitrate, allowing it to be recycled
through the food web. The two major natural sources of new
nitrogen entering this cycle are nitrogen-fixing organisms and
lightning.
Nitrogen-fixing organisms include a relatively small number
of algae and bacteria. Many of them live free in the soil, but the
most important ones are bacteria that form close symbiotic
relationships with higher plants. Symbiotic nitrogen-fixing
bacteria such as the Rhizobia, for instance, live and work in
nodules on the roots of peas, beans, alfalfa, and other legumes.
These bacteria manufacture an enzyme that enables them to
convert gaseous nitrogen directly into plant usable forms.
Lightning may also indirectly transform atmospheric
nitrogen into nitrates, which rain onto soil.
Quantifying the rate of natural nitrogen fixation prior to
human alterations of the cycle is difficult but necessary for
evaluating the impacts of human-driven changes to the global
cycling of nitrogen. The standard unit of measurement for
analyzing the global nitrogen cycle is the teragram (abbreviated
Tg), which is equal to a million metric tons of nitrogen.
Worldwide, lighting, for instance, fixes less than 10 Tg of
nitrogen per year—maybe even less than 5 Tg. Microbes are
the major natural suppliers of new biologically available
nitrogen. Before the widespread planting of legume crops,
terrestrial organisms probably fixed between 90 and 140 Tg of
nitrogen per year. A reasonable upper bound for the rate of
natural nitrogen fixation on land is thus about 140 Tg of
nitrogen per year.
During the past century, human activities clearly have
accelerated the rate of nitrogen fixation on land, effectively
doubling the annual transfer of nitrogen from the vast but
unavailable atmospheric pool to the biologically available
forms. The major sources of this enhanced supply include
industrial processes that produce nitrogen fertilizers, the
combustion of fossil fuels, and the cultivation of soybeans,
peas, and other crops that host symbiotic nitrogen-fixing
bacteria. Furthermore, human activity is also speeding up the
release of nitrogen from long-term storage in soils and organic
matter.
Industrial fixation of nitrogen for use as fertilizer currently
totals approximately 80 Tg per year and represents by far the
largest human contribution of new nitrogen to the global cycle.
That figure does not include manures and other organic
nitrogen fertilizers, which represent a transfer of already-fixed
nitrogen from one place to another rather than new fixation.
...
Until the late 1970s, most industrially produced fertilizer
was applied in developed countries. Use in these regions has
now stabilized while fertilizer applications in developing
countries have risen more dramatically. The momentum of
human population growth and increasing urbanization ensures
that industrial fertilizer production will continue at high and
likely accelerating rates for decades in order to meet the
escalating demand for food.