Principles of terrestrial ecosystem ecology.pdf

58 3. Geology and Soils
loosely bound ions on the exchange complex
with ions that bind more tightly. In this way
monovalent cations such as Na + , ammonium ion
(NH4 + ), and K + and anions such as chloride ion
(Cl - ) and nitrate ion (NO3 - ) are readily
released from the exchange complex into the
soil solution and are particularly prone to
leaching loss. The maintenance of charge
balance of soil solutions requires that the leaching
of negatively charged anions be accompanied
by an equal charge of positive ions
(cations). Inputs of sulfuric acid (H2SO4) in acid
rain therefore increase leaching losses of
readily exchangeable base cations like Na + ,
NH4 + , and K + , which leach downward with
sulfate ion (SO4 2- ).
Materials can also be lost from soils as gases.
Gas emissions depend on the rate of production
of the gas by microbes, the diffusional
paths through soils, and the exchange at the
soil–air interface (Livingston and Hutchinson
1995). The controls over these losses are discussed
in Chapter 9.
Soil Horizons and
Soil Classification
Ecosystem differences in additions, transformations,
transfers, accumulations, and losses
give rise to distinct soils and soil profiles. Soils
include organic, mineral, gaseous, and aqueous
constituents arranged in a relatively predictable
Soil
O
O i Organic, slightly decomposed
O e Organic, moderately decomposed
O a Organic, highly decomposed
A Mineral, mixed with humus, dark colored
E
B
C
R
Horizon of maximum leaching of silicate
clays, Fe, Al oxides, etc.
Zone of Fe and Al accumulation
Zone of least weathering and accumulation;
contains unweathered parent material
Bedrock
vertical structure. The number and depth of
horizons (layers) and the characteristics of each
layer in a soil profile vary widely among soils.
Nonetheless, a series of typical horizons can be
described for many soils (Fig. 3.9). The organic
horizon, or O horizon, consists of organic material
that accumulates above the mineral soil.
This layer of organic material is derived from
the litter of dead plants and animals. The O
horizon can be subdivided based on the degree
of decomposition that the majority of material
has undergone, with the lower portion of the O
horizon being more decomposed. The A
horizon is the uppermost mineral soil horizon.
Being adjacent to the organic layers, it typically
contains substantial quantities of organic
matter and is therefore dark in color.The O and
A horizons are the zones of most active plant
and microbial processes and therefore have
highest nutrient supply rates (see Chapter 9).
Many soils in wet climates have an E horizon
beneath the A horizon that is strongly leached.
Most clay minerals and iron and aluminum
oxides have been leached from the horizon,
leaving behind resistant minerals like quartz, in
addition to sand and silt-size particles. The B
horizon beneath the A and E horizons is the
zone of maximum accumulation of iron and
aluminum oxides and clays. Salts and precipitates
sometimes also accumulate here, especially
in arid and semiarid environments. The C
horizon lies beneath the A and B horizons.
Although it may accumulate some of the
Figure 3.9. A generic soil profile showing
the major horizons that are formed during
soil development. Density of dots reflects
concentration of soil organic matter.