Environmental Engineering Reference
In-Depth Information
Conversion or “fixation” of atmospheric nitrogen has been deliberately altered by
humans for the production of fertilizer to stimulate crop production and inadvertently via
the combustion of fossil fuels. Natural fixation results from lightning and the activities of
specialized organisms (discussed later). Human activities have doubled global N fixation,
mostly since 1950 (
Figure 7.1
). The increase has been driven by increases in fertilizer use
to support food production and by increases in fossil fuel combustion, which inadvertently
converts (or fixes) N
2
into reactive N (
Figure 7.2
). The increase in human nitrogen use has
occurred more recently and quickly than other major human alterations of global pro-
cesses (
Figure 7.1
). A major technological advance facilitating human nitrogen use was the
development of the Haber-Bosch process for chemically fixing nitrogen to the atmosphere.
In this process, N
2
is reacted with hydrogen gas (H
2
) over an iron or ruthenium catalyst to
produce ammonia. The process was originally developed to facilitate the production of
explosives early in the twentieth century and was adopted for fertilizer use decades later.
Widespread increases in the cultivation of N-fixing crops have also facilitated the increase
in human nitrogen use. Because food production and fossil fuel combustion are high in
the United States, the alteration of the U.S. N cycle is much more dramatic than the global
average. Changes in the nitrogen cycle are currently much more dynamic in the develop-
ing world, for example China and India, which is experiencing rapid increases in both fer-
tilizer use and fossil fuel combustion.
Extra reactive N added to the atmosphere by humans via fossil fuel combustion and as
fertilizer cascades through the environment and causes a series of biological, chemical,
and physical changes in ecosystems (
Figure 7.3
). For example, some of the fertilizer that is
applied to agricultural fields leaves those fields in surface runoff or ground-water flow.
This N becomes a drinking water pollutant and cascades through the environment in
streams, rivers, estuaries, and eventually in the sea. This N stimulates the productivity of
primary producers in aquatic ecosystems just as it does in crop fields leading to eutrophi-
cation (a process where water bodies receive excess nutrients that stimulate excessive
plant and algal growth), especially in coastal ecosystems. As the excessive growth decays
in the bottom waters of these ecosystems, oxygen is consumed, leading to hypoxic dead
zones in ecosystems across the world, from the Gulf of Mexico to the Chesapeake Bay to
the Baltic and South China seas.
FIGURE 7.1
The time course of selected
human actions that have effects at the global
scale. Note that human manipulation of the
global N cycle has been the most recent and
drastic component of global change.
75
(From
Vitousek et al. 1997
.)
50
Deforestation
CO
2
release
Industrial
N fertilizer
25
Human
population
1700
1800
1900
1975
