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Too
much of a good thing
What
do you think is the worst pollutant of the Chesapeake
Bay and the coastal environment_ Well, the topic of today's
Watershed Radio program already gives it away: It is nitrogen.
Not many people would think about nitrogen as a major
pollutant, but nitrogen, coming from treated sewage, polluted
air, and the runoff of fertilizer and manure, is polluting
the Chesapeake Bay and its watershed.
As
a basic building block of plant and animal proteins, nitrogen
is a nutrient essential to all forms of life. Until recently,
the supply of nitrogen available to plants—and ultimately
to animals—has been quite limited. Although it is
the most abundant element in the atmosphere, nitrogen
from the air cannot be used by plants until it is chemically
transformed, or fixed, into ammonium or nitrate compounds
that plants can metabolize.
In
nature, only certain bacteria and algae (and, to a lesser
extent, lightning) have this ability to fix atmospheric
nitrogen, and the amount that they make available to plants
is comparatively small. Other bacteria break down nitrogen
compounds in dead matter and release it to the atmosphere
again. As a consequence, nitrogen is a precious commodity—a
limiting nutrient—in most undisturbed natural systems.
All
that has changed in the past several decades. Driven by
a massive increase in the use of fertilizer, the burning
of fossil fuels, and an increase in land clearing and
deforestation, the amount of nitrogen available for uptake
has more than doubled since the 1940s. In other words,
human activities now contribute more to the global supply
of fixed nitrogen each year than natural processes do.
The
table below compares the contribution to fixed nitrogen
by human activities and by natural processes and indicates
that human-generated nitrogen totals about 210 million
metric tons per year, while natural processes contribute
about 140 million metric tons.
|
Global sources of biologically available (fixed) nitrogen |
| Sources
related to human activities |
|
Release
of fixed nitrogen per year (teragrams)
|
| Fertilizer |
|
80
|
| Legumes
and other plants |
|
40
|
| Fossil
fuels |
|
20
|
| Biomass
burning |
|
40
|
| Wetland
draining |
|
10
|
| Land
clearing |
|
20
|
| |
|
|
|
Total
from human sources
|
|
210
|
| |
|
|
| Natural
sources |
|
|
| Soil
bacteria, algae, lightning, etc. |
|
140
|
| |
|
|
|
1
Teragram = 1000,000,000,000 grams.
Source:
Peter M. Vitousek et al., "Human Alteration
of the Global Nitrogen Cycle: Causes and Consequences,"
Issues in Ecology, No. 1 (1997), pp. 4-6, as described
in Nutrient
Overload: Unbalancing the Global Nitrogen Cycle ,
a publication by the World Resources Institute.
|
This
influx of extra nitrogen has caused serious distortions
of the natural nutrient cycle and is threatening air and
water quality and disrupting the health of terrestrial
and aquatic ecosystems. Although terrestrial ecosystems
are vulnerable to the extra nitrogen input, aquatic ecosystems
in lakes, rivers, and coastal estuaries have probably
suffered the most so far.
In
these aquatic systems, excess nitrogen (as well as phosphorus)
stimulates the growth of algae and other aquatic plants
and, in some circumstances, stimulates toxic algal blooms.
When the algae die, their decay uses so much oxygen that
it results in low levels of dissolved (or available) oxygen
in the water, a situation, called anoxia, that can suffocate
other aquatic organisms.
The
nitrogen overload also effects air quality when the nitrogen-containing
gases nitric oxide and nitrous oxide are released into
the air, either from fossil fuel burning, land clearing,
or agriculture-related activities. Nitric oxide, for example,
is one of the ingredients of smog and acid rain, and nitrous
oxide is a long-lived greenhouse gas that traps some 200
times more heat than carbon dioxide.
References
and further reading
Nutrient
Overload: Unbalancing the Global Nitrogen Cycle.
Information from the World Resources Institute and
the main source of the information in this Watershed Radio
program.
Human
Alteration of the Global Nitrogen Cycle: Causes and Consequences.
An overview of the scientific understanding of human-driven
changes to the global nitrogen cycle and their consequences.
A publication from the Ecological Society of America.
The
Nitrogen Cycle.
Part of an online course in Physical Geography, by Michael
J. Pidwirny, Ph.D., Department of Geography, Okanagan
University College.
The
Tragedy of Fritz Haber.
An NPR program and article about Fritz Haber, a Nobel
laureate who transformed world food production by inventing
the process for turning air into nitrogen fertilizer Without
it, the earth wouldn't be able to support its current
population. But the invention also has flooded the world
with pollution.
It's
not the oil, it's the nitrogen. An article by Tom
Horton in the
Baltimore Sun
about people's perceptions. What people believe they know
about pollution does not always coincide with the scientific
facts.
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