Principles of terrestrial ecosystem ecology.pdf

chemical weathering through their contribution
to soil acidity and their capacity to chelate ions.
In the chelation process, organic acids combine
with metallic ions, such as ferric iron (Fe 3+ ) and
aluminum (Al 3+ ), making them soluble and
mobile. Chelation lowers the concentration of
inorganic ions at the mineral surface, so dissolved
and primary mineral forms are no longer
in equilibrium with one another. This accelerates
the rate of weathering.
The physical and chemical properties of
rock minerals determine their susceptibility
to weathering and the chemical products that
result. Sedimentary rocks like shale that form
by chemical precipitation, for example, have
more basic cations like calcium (Ca 2+ ), sodium
(Na + ) and potassium (K + ) than does igneous
rock and tends to produce soils with a relatively
high pH and a high capacity to supply mineral
cations to plants. Igneous rocks weather in the
reverse order in which they crystallize during
formation (Birkeland 1999). Olivine, for
example, is one of the first minerals to crystallize
as magma cools. It has a high energy of
formation and weathers easily. Feldspar forms
and weathers more slowly than olivine, and
quartz is one of the last minerals to form
(explaining why it forms crystals) and is highly
resistant to weathering (Table 3.2). Secondary
minerals such as the silicate clay minerals and
iron and aluminum oxides are among the most
resistant minerals to weathering. Textural differences
in parent material also influence the
rate of chemical breakdown, with fine-grained
rocks weathering more slowly than coarsegrained
rocks.
Warm climates promote chemical weathering
because temperature speeds chemical reactions
Table 3.2. Stability of common minerals under
weathering conditions at Earth’s surface.
Most stable Fe 3+ oxides Secondary mineral
Al 3+ oxides Secondary mineral
Quartz Primary mineral
Clay minerals Secondary mineral
K + feldspar Primary mineral
Na + feldspar Primary mineral
Ca 2+ feldspar Primary mineral
Least stable Olivine Primary mineral
Data from Press and Siever (1986).
Development of Soil Profiles 55
(A) (B)
and
and
= Oxygen
= Silicon
(C) (D)
and = Hydroxyl
= Aluminum, magnesium, etc.
Figure 3.7. The molecular structure of a simple clay
layer. A, A tetrahedral unit. B, A tetrahedral sheet.
C, An octahedral unit. D, An octahedral sheet.
(Redrawn with permission from Clay Mineralology
by R.E. Grim, © 1968 McGraw-Hill Companies;
Grim 1968.)
by increasing the kinetic energy of reactants.
The activities of plants and microorganisms
are also more rapid under warm conditions.
Wet conditions promote weathering through
their direct effects on weathering reactions and
their effects on biological processes. Not surprisingly,
the hot wet conditions of humid
tropical climates yield the highest rates of
chemical weathering.
The secondary minerals formed in weathering
reactions play critical roles in soils and
ecosystem processes. In temperate soils, weathering
products include layered silicate clay minerals.
These small particles (less than 0.002mm)
are hydrated silicates of aluminum, iron, and
magnesium arranged in layers to form a crystalline
structure. Two types of sheets make up
these minerals: A tetrahedral sheet consists of
units composed of one silicon atom surrounded
by four atomic oxygen (O - ) groups (Fig.
3.7A,B). An octahedral sheet consists of units
having six oxygen (O - ) or hydroxide (OH - ) ions
surrounding an Al 3+ , magnesium ion (Mg 2+ ), or
Fe 3+ ion (Fig. 3.7C,D). Various combinations
of these sheets give rise to a wide variety of
clay minerals. Montmorillonite and illite, for
example, have 2:1 ratios of silica to aluminumdominated
layers. Kaolinite, a more strongly