Principles of terrestrial ecosystem ecology.pdf

tive with soil minerals. The productivity of
aquatic ecosystems is so sensitive to phosphorus
additions that even small additions in
groundwater can cause large changes in their
functioning (see Chapter 13).
Sulfur
Sulfur cycling is particularly complex because it
undergoes oxidation–reduction reactions, like
nitrogen, and has both gaseous and mineral
sources (Fig. 9.10). Rock weathering, which is
the primary natural source of sulfur in most
ecosystems, is being increasingly supplemented
by atmospheric inputs in the form of acid rain.
Combustion of fossil fuels produces gaseous
sulfur dioxide (SO2), which dissolves in cloud
droplets to produce sulfuric acid (H2SO4), a
strong acid that is responsible for much of the
lake acidification downwind of industrial areas
(see Chapter 15). Sulfur in plant tissues is both
carbon and ester bonded, so microbial mineralization
includes immobilization and release
processes similar to those of nitrogen and phosphorus.
Like nitrogen, inorganic sulfur undergoes
oxidation–reduction reactions and is
therefore sensitive to oxygen availability in the
environment. In anaerobic soils, sulfate acts as
an electron acceptor that allows microbes to
metabolize organic carbon for energy, with
hydrogen sulfide being produced as a byproduct.
In aerobic environments, however,
reduced sulfur can be an important energy
source for bacteria. The productivity associated
with deep-sea vents, for example, is based
entirely on the oxidation of hydrogen sulfide
(H2S) from the vents. Sulfur is a component
of most enzymes, including the nitrogenase of
nitrogen fixers, so low availability of sulfur in
highly weathered soils in unpolluted areas can
limit nitrogen inputs to ecosystems and therefore
plant production and nutrient turnover.
Superphosphate fertilizer has a high sulfur concentration,
so vegetation responses to application
of phosphate fertilizers in some ecosystems
may include a response to sulfur (Eviner et al.
2000). Sulfur compounds in the atmosphere
play critical roles as aerosols, which increase the
albedo of the atmosphere and therefore cause
climatic cooling (see Chapter 2).
Essential Cations
Other Element Cycles 219
Rock weathering is the primary avenue for
element inputs of potassium, calcium, magnesium,
and manganese, the cations required in
largest amounts by plants. As with nitrogen,
phosphorus, and sulfur, the quantities of these
cations cycling in ecosystems from soils to
plants and back to soils are much larger than
are annual inputs to and losses from ecosystems.
The availability of cations in the soil solution
is largely governed by exchange reactions
and depends on the cation exchange capacity of
the soil and its base saturation (see Chapter 3),
which, in turn, is influenced by parent material
and weathering characteristics. Calcium is an
important structural component of plant and
fungal cell walls. Its release and cycling therefore
depends on decomposition in a way somewhat
similar to that of nitrogen and phosphorus
(Fig. 9.11). Potassium, on the other hand, occurs
primarily in cell cytoplasm and is released
through the leaching action of water moving
through live and dead organic material.
Magnesium and manganese are intermediate
between calcium and potassium in their cycling
characteristics.
Potassium limits plant production in some
ecosystems, but calcium concentration in the
soil solution of most ecosystems is so high
that it is actively excluded by plant cells during
the uptake process (see Chapter 8). Availability
of calcium and other cations may be low
enough to limit plant production on the old,
highly weathered tropical soils or where acid
rain has leached most available calcium from
soils.
These cations have no gaseous phase, but
atmospheric transfers of these elements (and
of essential micronutrients) in dust can be an
important pathway of loss from deserts and
agricultural areas that experience wind erosion
and an important input to the open ocean and
to ecosystems on highly weathered parent
materials. Cations can also be lost via leaching.
Nitrate or other anions that are leached from
ecosystems must be accompanied by cations to
maintain electrical neutrality. Thus high nitrate
leaching rates that occur in nitrogen-saturated
sites or as a result of excessive nitrogen fertilization
are accompanied by high losses of