Principles of terrestrial ecosystem ecology.pdf

Nitrogen output (g m -2 yr -1 )
5
4
3
2
1
0
North America
Europe
0 1 2 3 4
Nitrogen deposition (g m -2 yr -1 )
Figure 9.9. Comparisons of inputs from nitrogen
deposition and nitrogen outputs in solution from
forests of North America and Europe. There is a
strong relationship between inputs and outputs in
nitrogen-saturated ecosystems. (Data from Tietema
and Beier 1995, Fenn et al. 1998.)
often has nitrate concentrations that exceed
public health standards.
Nitrogen leached from terrestrial ecosystems
moves in groundwater to lakes and rivers. The
movement of nitrate to the North Atlantic
Ocean from major rivers has increased 6- to 20fold
in the past century (Howarth et al. 1996),
primarily due to increased inputs of fertilizer,
atmospheric deposition, nitrogen fixation by
crops, and food imports (see Chapter 15).
Nitrate in coastal marine systems frequently
increases productivity and detritus accumulation.
The resulting stimulation of decomposition
can reduce oxygen concentrations
sufficiently to kill fish, particularly in winter,
when primary production is temperature
limited (see Chapter 14). The nitrogen loading
from agricultural and urban systems in the
Mississippi drainage, for example, has produced
a “dead zone” where this river enters the
Gulf of Mexico (Mitsch et al. 2001).
Solutions that move through the soil must
maintain a balanced charge, with negatively
charged ions like nitrate balanced by cations.
Therefore, every nitrate ion that leaches from
soil carries with it a cation such as calcium,
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Other Element Cycles 215
potassium, and ammonium to maintain charge
balance. When cation loss by leaching exceeds
the rate of cation supply by weathering plus
deposition, the net loss of cations can lead to
cation deficiency (Driscoll et al. 2001). After
these nutrient cations are depleted, nitrate
takes with it H + and/or Al 3+ , which are deleterious
to downstream ecosystems. Nitrification
also generates acidity:
2NH4 + + 3O2 Æ 2NO2 - + H2O + H + (9.5)
The hydrogen ion released in this reaction
exchanges with other ions on cation exchange
sites in the soil, making these cations more vulnerable
to leaching loss.
Erosional Losses
Erosion is a natural pathway of nitrogen loss
that often increases dramatically after land use
changes. As with leaching, erosional losses of
nitrogen include both organic and inorganic
forms, although organic forms associated with
soil aggregates and particles are most important.
In some ecosystems, especially those on
unstable slopes or in areas exposed to high
winds, erosion is a dominant natural pathway of
nitrogen loss.
Other Element Cycles
Phosphorus
Weathering of primary minerals is the major
source of new phosphorus to ecosystems. In
contrast to nitrogen, whose major source is the
atmosphere, phosphorus enters ecosystems primarily
by weathering of rocks (Fig. 9.10). The
weathering of phosphorus-containing apatite
by the carbonic acid generated from soil respiration,
for example, releases phosphorus in
available forms (Eq. 9.6) that can be taken up
directly by plants or microorganisms or be
adsorbed or precipitated (see Chapter 3).
Ca5(PO4)3 + 4H2CO3 Æ
5Ca 2+ + 3HPO4 2- + 4HCO3 + H2O (9.6)
Phosphorus inputs from weathering depend
on the mineralogy of the parent material, the