Principles of terrestrial ecosystem ecology.pdf

Organic
carbon (%)
Erosion
likely
grained materials downslope and deposit them
at lower locations. Depositional areas at the
base of slopes and in valley bottoms therefore
tend to have deep fine-textured soils with a high
soil organic content (Fig. 3.3) and high waterholding
capacity. Depositional areas supply
more soil resources to plant roots and microbes
and provide greater physical stability than do
higher slope positions. For these reasons valley
bottoms typically exhibit higher rates of most
ecosystem processes than do ridges or shoulders
of slopes. Soils in lower slope positions
in sagebrush ecosystems, for example, have
greater soil moisture, higher soil organic matter
content, and higher rates of nitrogen mineralization
and gaseous losses than do upslope soils
(Burke et al. 1990, Matson et al. 1991).
Slope position also determines patterns of
snow redistribution in cold climates, with
deepest accumulations beneath ridges and in
the protected lower slopes. These differential
accumulations alter effective precipitation and
length of growing season sufficiently to influence
plant and microbial processes well into the
summer.
Finally, the aspect of a slope influences solar
input (see Chapter 2) and therefore soil temperature,
rates of evapotranspiration, and soil
moisture. At high latitudes and in wet climates,
these differences in soil environment reduce
rates of decomposition and mineralization on
poleward-facing slopes (Van Cleve et al. 1991).
At low latitudes and in dry climates, however,
the greater retention of soil moisture on
4
3-4
2-3
1-2
Figure 3.3. Relationship between hillslope position,
likelihood of erosion or deposition, and soil organic
carbon concentration. (Redrawn with permission
from Oxford University Press; Birkeland 1999.)
Controls over Soil Formation 49
poleward-facing slopes allows a longer growing
season and supports forests, whereas slopes
facing the equator are more likely to support
desert or shrub vegetation (Whittaker and
Niering 1965).
Time
Many soil-forming processes occur slowly, so
the time over which soils develop influences
their properties. Rocks and minerals are
weathered over time, and important nutrient
elements are transferred among soil layers or
transported out of the ecosystem. Hillslopes
erode, and valley bottoms accumulate materials,
and biological processes add organic matter
and critical nutrient elements like carbon
and nitrogen. Phosphorus availability is high
early in soil development and becomes progressively
less available over time due to its
losses from the system and to its fixation in
mineral forms that are unavailable to plants
(Fig. 3.4) (Walker and Syers 1976). This process
required millions of years of soil development
in Hawaii, despite a warm moist climate (Crews
et al. 1995) and resulted in a change from nitrogen
limitation of plant growth on young soils to
phosphorus limitation on older soils (Vitousek
et al. 1993).
Some changes in soil properties happen
relatively quickly. Retreating glaciers and river
floodplains often deposit phosphorus-rich till. If
seed sources are available, these soils are colonized
by plants with symbiotic nitrogen-fixing
microbes, allowing such ecosystems to accumulate
their maximum pool sizes of carbon and
nitrogen within 50 to 100 years (Crocker and
Major 1955, Van Cleve et al. 1991). Other soilforming
processes occur slowly. Young marine
terraces in coastal California have relatively
high phosphorus availability but low carbon
and nitrogen content. Over hundreds of thousand
of years, these terraces accumulate organic
matter and nitrogen, causing a change from
coastal grassland to productive redwood forest
(Jenny et al. 1969). Over millions of years, silicates
are leached out, leaving behind a hardpan
of iron and aluminum oxides with low fertility
and seasonally anaerobic soils. The pygmy
cypress forests that develop on these old ter-