Principles of terrestrial ecosystem ecology.pdf

54 3. Geology and Soils
Additions to Soils
Additions to the soil system can come from
outside or inside the ecosystem. Inputs from
outside the ecosystem include precipitation and
wind, which deposit ions and dust particles,
and floods and tidal exchange, which deposit
sediments and solutes (see Chapter 9). The
source of these materials determines their
size distribution and chemistry, leading to the
development of soils with specific textural and
chemical characteristics. Organisms within the
ecosystem add organic matter and nitrogen to
the soil, including the aboveground and belowground
portions of plants, animals, and soil
microbes.
Soil Transformations
Within the soil, materials are transformed
through an interaction of physical, chemical,
and biological processes. Freshly deposited
dead organic matter is transformed in the soil
by decomposition to soil organic matter,
releasing carbon dioxide and nutrients such as
nitrogen and phosphorus (see Chapter 7). More
recalcitrant organic compounds undergo
physicochemical interactions with soil minerals
that contribute to the long-term storage of soil
organic matter.
Weathering is the change of parent rocks
and minerals to produce more stable forms.
This occurs when rocks and minerals become
exposed to physical and chemical conditions
different from those under which they formed
(Ugolini and Spaltenstein 1992). Weathering
involves both physical and chemical processes
and is influenced by characteristics of the
parent material and by temperature, moisture,
and the activities of organisms. Physical weathering
is the fragmentation of parent material
without chemical change. This can occur
when rocks are fractured by earthquakes or
when stresses are relieved due to erosional loss
of the weight of overlying rock and soil. In
addition, soil particles and rock fragments
are abraded by wind or are ground against
one another by glaciers, landslides, or floods.
Rocks also fragment when they expand and
contract during freeze–thaw, heating–cooling,
or wetting–drying cycles or when roots grow
into rock fissures. Fire is a potent force of
physical weathering because it rapidly heats
the rock surface to a high temperature while
leaving the deeper layers cool. Physical weathering
is especially important in extreme and
highly seasonal climates. Wherever it occurs, it
opens channels in rocks for water and air to
penetrate, increasing the surface area for chemical
weathering reactions.
Chemical weathering occurs when parent
rock materials react with acidic or oxidizing
substances, usually in the presence of water.
During chemical weathering, primary minerals
(minerals present in the rock or unconsolidated
parent material before chemical changes have
occurred) are dissolved and altered chemically
to produce more stable forms, ions are released,
and secondary minerals (products that are
formed through the reaction of materials
released during weathering) are formed. Some
primary minerals can be hydrolyzed by water,
producing new minerals plus ions in solution.
Hydrolysis reactions, however, typically include
both water (H 2O) and an acid. Carbonic acid
(H2CO3) is the most important acid involved in
chemical weathering. The CO2 concentration in
most soils is 10- to 30-fold higher than in air,
due to the low diffusivity of gases in soil and the
respiration of plants, soil animals, and microorganisms.
Weathering rates are particularly high
adjacent to roots because of the high rates
of biological activity and CO2 production in
the rhizosphere. Carbon dioxide dissolves and
reacts with water to form carbonic acid, which
then ionizes to produce a hydrogen ion (H + )
and a bicarbonate ion (HCO3 - ). Carbonic
acid, for example, attacks potassium feldspar
(KAlSi3O8), which is converted into a secondary
mineral, kaolinite (Al2Si2O5(OH)4), by
the removal of soluble silica (SiO2) and potassium
ion (K + ) (Eq. 3.1). Kaolinite can, under the
right conditions, undergo another dissolution
to form another secondary mineral gibbsite
(Al(OH)3).
+
-
2KAlSi3O8 + 2( H + HCO3 )+ H2O Æ
+
-
Al 2Si 2O5( OH) 4 + 4SiO2 + 2K + 2HCO3
(3.1)
Plant roots and microbes secrete many
organic acids into the soil, which influence