Principles of terrestrial ecosystem ecology.pdf

y roots and bacteria are important sources
of organic matter that binds soil particles
together. Fungal hyphae contribute strongly to
aggregation in many soils. For these reasons, the
loss of soil organic matter and its associated
microbes can lead to a loss of soil structure,
which contributes to further soil degradation.
Earthworms and other soil invertebrates contribute
to aggregate formation by ingesting soil
and producing feces that retain a coherent
structure. Plant species and their microbial
associates differ in the capacity of their exudates
to form aggregates, so soil texture, organic
matter content, and species composition all
influence soil structure.
The cracks and channels between aggregates
are important pathways for water infiltration,
gaseous diffusion, and root growth, thus affecting
water availability, soil aeration, oxidation–reduction
processes, and plant growth.The
fine-scale heterogeneity created by soil aggregates
is critical to the functioning of soils. Slow
gas diffusion through the partially cemented
pores within aggregates creates anaerobic conditions
immediately adjacent to aerobic surfaces
of soil pores.This allows the occurrence in
well-drained soils of anaerobic processes such
as denitrification, which requires the products
of aerobic processes (nitrification, in this case)
(see Chapter 9).
Compaction by animals and machinery fills
the cracks and pores between aggregates.
Plowing reduces aggregation through mechanical
disruption of aggregates and through loss
of soil organic matter and associated cementing
activity of microbial exudates and fungal
hyphae (Fisher and Binkley 2000). The loss of
soil structure through compaction prevents
rapid infiltration of rainwater and leads to
increased overland flow and erosion.
Bulk density is the mass of dry soil per unit
volume, usually expressed in grams per cubic
centimeter (gcm -3 ; equivalent to megagrams
per cubic meter, or Mgm -3 ). Bulk density varies
with soil texture and soil organic matter
content. Bulk densities of mineral soils (1.0 to
2.0gcm -3 ) are typically higher than those of
organic soil horizons (0.05 to 0.4gcm -3 ). Finetextured
soils have higher internal surface area
and more pore space than coarser-textured
Soil Properties and Ecosystem Functioning 63
soils, and thus their bulk densities tend to be
lower. If they are compacted, however, bulk
densities of clay soils can be higher than those
of coarse-textured soils. Bulk density strongly
influences the nutrient and water characteristics
of a site. Organic soils, for example,
frequently have highest concentrations (percent
of dry mass) of carbon in surface horizons with
low bulk density but the greatest quantities
(grams per cubic centimeter) of carbon at
depth, where bulk density is greater. The quantity
of nutrient per unit volume is calculated by
multiplying the percentage concentration of the
nutrient times soil bulk density. Volumetric
nutrient content is generally more relevant
than nutrient concentration in describing the
quantity of nutrients directly available to plants
and microbes.
Water is a critical resource for most ecosystem
processes. In soils, water is held in pore
spaces as films of water adsorbed to soil particles.
The soil is water-saturated when all pore
spaces are filled with water. Under these conditions
water drains under the influence of
gravity (saturated flow) until, often after
several days, the adhesive forces that hold
water in films on soil particles equals the
gravitational pressure. At this point, called field
capacity, water no longer freely drains.
At water contents below field capacity, water
moves through the soil by unsaturated flow in
response to gradients of water potential—that
is, the potential energy of water relative to pure
water (see Chapter 4).When plant roots absorb
water from the soil to replace water that is lost
in transpiration, there is a reduction in the
thickness of water films adjacent to roots, which
causes the remaining water to adhere more
tightly to soil particles. The net effect is to
reduce the soil water potential at the root
surface. Water moves along water films through
the soil pores toward the root in response to
this gradient in water potential. Plants continue
to transpire, and water continues to move
toward the root until some minimal water
potential is reached, when roots can no longer
remove water from the particle surfaces. This
point is called the permanent wilting point.
Water-holding capacity is the difference in
water content between field capacity and per-