Agriculture Reference
In-Depth Information
The Forest System
A tree represents a system within a system. It can be part of a forest ecosystem,
where it depends on nutrients provided by the air and soil. The soil receives its
nutrients through abiotic and biotic processes, such as nitrates from lightning,
nitrogen-fixing bacteria in legumes' root nodules, and the breakdown of de-
tritus by aerobes and anaerobes on the forest floor. The nitrogen cycle is quite
complex (see Fig. B7.3). Basically, numerous simultaneous chemical reactions
are taking place, so the forest ecosystem is a balance of various chemical
forms of nitrogen (and phosphorus, sulfur, and carbon, for that matter). The
chemical reactions in a nutrient cycle consist of biochemical processes whereby
organisms take simpler nitrogen compounds, including microbial fixation of
molecular nitrogen (N 2 ) from the atmosphere and form amino acids in the
tissues of plants and animals. In the opposite direction, mineralization is the
process by which organic matter is reduced or oxidized to mineral forms, such
as ammonia, ammonium hydroxide, nitrite, and nitrate. Note that the gases
at the top of the figure include those that are important in air pollution. For
example, NO is one of the compounds involved in the photochemistry that
leads to formation of the pollutant ozone (O 3 ) in the troposphere. Note also
N 2
N 2
NH 3
N 2 O
NO
Air
Symbiotic
Nonsymbiotic
Fixation of nitrogen
Plant uptake
Soil
Nitrification
(aerobic
processes)
Organic matter in
detritis and dead organisms
Mineralization
+
-
NO 3 -
NH 3 /NH 4
NH 2 OH
NO 2
-
N 2
N 2 O
NO
NO 2
Dentrification (anaerobic processes)
Figure B7.3 Nitrogen cycling in a forest ecosystem.
FromD. A. Vallero, Environmental Contaminants: Assessment and Control , Elsevier Academic Press, Burlington,
MA, 2004.
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