SAT Reading - Khan Diagnostic Quiz level 4 - reading 16

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
_Line oxygen, carbon, and hydrogen as the most common chemical
₅ lement 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
₁₀ 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
₁₅ 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.
₂₀ 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
₂₅ 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-
₃₀ 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
₃₅ nitrogen-fixing bacteria such the 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.
₄₀ 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
₄₅ 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, lightning, for instance, fixes less than

10 Tg of nitrogen per year - maybe even less than 5 Tg.
₅₀ 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
₅₅ 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
₆₀ 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
₆₅ 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
₇₀ 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.
₇₅ 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
₈₀ ensures that industrial fertilizer production will continue at

high and likely accelerating rates for decades in order to meet

the escalating demand for food.