Principles of terrestrial ecosystem ecology.pdf

66 3. Geology and Soils
ferent cations are held to the exchange
complex. In general, cations occupy exchange
sites and displace other ions in the sequence
H(Al 3+ ) > H + > Ca 2+ > Mg 2+ > K +
ª NH4 + > Na + (3.4)
so leached soils tend to lose Na + and NH4 + but
retain Al 3+ and H + . This displacement series is a
consequence of differences among ions in
charge and hydrated radius. Ions with more
positive charges bind more tightly to the
exchange complex than do ions with a single
charge. Ions with a smaller hydrated radius
have their charge concentrated in a smaller
volume and tend to bind tightly to the exchange
complex.
Minerals like the iron and aluminum oxides
found in many tropical soils have surface
charges that vary between positive and negative,
depending on pH. At the low pH conditions
typical of these soils, the net charge is
sometimes positive (Uehara and Gillman
1981), so they attract anions, creating an anion
exchange capacity. As with cations, anion
absorption depends on the concentration of
anions and their relative capacities to be held
or to displace other anions. Anions generally
occupy exchange sites and displace other ions
in the sequence
PO4 3- > SO4 3- > Cl - > NO3 - (3.5)
so leached soils tend to lose NO3 - and Cl - but
retain phosphate (PO4 3- ) and sulfate (SO4 3- ).
This retention reflects both anion exchange and
the formation of covalent bonds that are not
readily broken.
The high CEC and base saturation found in
many soils, especially in many temperate soils,
provide buffering capacity that keeps the soils
from becoming acid. When additional H + is
added to the system in solution (e.g., in acid
rain), it exchanges with cations that were held
on cation exchange sites on clay minerals and
soil organic matter. Buffering capacity allows
the pH in forest soils to remain relatively
constant for long periods despite chronic exposure
to acid rain. When the buffering capacity
is exceeded, the soil pH begins to drop, which
can solubilize aluminum hydroxides (Al(OH)x),
Al 3+ , and other cations, with potentially toxic
effects on both terrestrial and downstream
aquatic ecosystems (Schulze 1989, Aber et al.
1998). In many tropical soils, the relatively low
CEC does not function as efficiently to buffer
soil solution chemistry. Additions of acids to
these already acidic unbuffered systems
releases aluminum in solution more readily,
making these soils potentially toxic to many
plants and microbes.
Summary
Five state factors control the formation and
characteristics of soils. Parent material is generated
by the rock cycle, in which rocks are
formed, uplifted, and weathered to produce the
materials from which soil is derived. Climate is
the factor that most strongly determines the
rates of soil-forming processes and therefore
rates of soil development. Topography modifies
these rates at a local scale through its effects on
microclimate and the balance between soil
development and erosion. Organisms also
strongly influence soil development through
their effects on the physical and chemical environment.
Time integrates the impact of all state
factors in determining the long-term trajectory
of soil development. In recent decades, human
activities have modified the relative importance
of these state factors and substantially altered
Earth’s soils.
The development of soil profiles represents
the balance between profile development, soil
mixing, erosion, and deposition. Profile development
occurs through the input, transformation,
vertical transfer, and loss of materials from
soils. Inputs to soils come from both outside the
ecosystem (e.g., dust or precipitation inputs)
and inside the ecosystem (e.g., litter inputs).
The organic matter inputs are decomposed to
produce CO2 and nutrients or are transformed
into recalcitrant organic compounds. The carbonic
acid derived from CO2 and the organic
acids produced during decomposition convert
primary minerals into clay-size secondary
minerals, which have greater surface area and
cation exchange capacity. Water moves these
secondary minerals and the soluble weathering
products down through the soil profile until