Principles of terrestrial ecosystem ecology.pdf

conductivity of soils. Groundwater and shallow
subsurface flow dissolve and remove ions and
small particles. At the opposite extreme, overland
flow causes erosion primarily by surface
sheet wash, rills, and rain splash. This typically
occurs in arid and semiarid soil-mantled landscapes
or on disturbed ground. Overland flow
rates of 0.15 to 3cms -1 are sufficient to suspend
clay and silt particles and move them downhill
(Selby 1993). As water collects into gullies,
its velocity, and therefore erosion potential,
increases. Vegetation and litter layers greatly
increase infiltration into the soil by reducing
the velocity with which raindrops hit the
soil, thereby preventing surface compaction by
the raindrops. Vegetated soils are also less
compact because roots and soil animals create
channels in the soil. In these ways vegetation
and a litter layer substantially increase infiltration
and therefore groundwater and subsurface
flow.
Wind is an important agent of erosion in
areas where wind speeds are high at the soil
surface, for example, where vegetation removal
exposes the soil surface to strong winds. Some
Figure 3.6. Processes leading to additions,
transformations, transfers, and
losses of materials from soils. Silica is
H 4SiO4. (Redrawn with permission from
Soils and Geomorphology by Peter W.
Birkeland, © 1999 Oxford University
Press, Inc.; Birkeland 1999.)
Ground surface
Soil
Development of Soil Profiles 53
agricultural areas in China have lost meters of
soil to wind erosion and have become an important
source of iron to the phytoplankton in the
Pacific Ocean (see Chapter 10).
Glaciers are an important erosional pathway
in cold, moist climates. Glacial rivers often
carry large sediment loads and produce a
braided river valley with meandering stream
channels that are important locations of
primary succession.
Erosion in one location must be balanced
by deposition elsewhere. Deposition can range
from slow rates of dust or loess input to
siltation events during floods to massive
moraines or debris accumulations at the base of
slopes.
Development of Soil Profiles
Soils develop through the addition of materials
to the system, transformation of those materials
in the system, transfer down and up in the
soil profile, and loss of materials from the
system (Fig. 3.6).
ADDITIONS
Precipitation (including
ions and solid particles);
organic matter
TRANSFORMATIONS
Organic matter humus
hydrous oxides
Primary minerals clays
ions, H SiO 4 4
TRANSFERS
Humus
compounds,
clays,
ions, H 4 SiO 4
LOSSES
Ions,
H4SiO4 TRANSFERS
Ions, H 4 SiO 4