Principles of terrestrial ecosystem ecology.pdf

64 3. Geology and Soils
manent wilting point (see Fig. 4.5). Waterholding
capacity is substantially enhanced by
presence of clay and soil organic matter
because of the large surface area of these materials.
The water-holding capacity of an organic
soil might, for example, be 300% (300g H2O per
100g dry soil), while that of a clay soil may be
30% and that of a sandy soil could be less than
20%. On a volumetric basis, water-holding
capacity is normally highest in loam soils. One
consequence of this difference is that, for a
given amount of rainfall, sandy soils are wetted
more deeply than clay soils but retain less water
in soil horizons that are accessible to plants.
The water-holding characteristics of soils help
determine the amount of water available for
plant uptake and growth and for microbial
processes, including decomposition and nutrient
cycling and loss.
Oxidation–reduction reactions involve the
transfer of electrons from one reactant to
another, yielding chemical energy that can be
used by organisms (Lindsay 1979). In these
reactions, the energy source gives up one or
more electrons (oxidation). These electrons are
transferred to electron acceptors (reduction).A
handy mnemonic is: “LEO the lion says GER,”
where LEO stands for loss of electrons—oxidation,
and GER stands for gain of electrons—
reduction. Redox potential is the electrical
potential of a system due to the tendency of
substances in it to lose or accept electrons
(Schlesinger 1997, Fisher and Binkley 2000).
There is a wide range of redox potentials
among soils due to their ionic and chemical
compositions. One important set of redox reactions,
which occurs inside the mitochondria of
live eukaryotic cells, is the transfer of electrons
from carbohydrates through a series of reactions
to oxygen.This series of reactions releases
the energy needed to support cellular growth
and maintenance. Many other redox reactions
occur in the cells of soil organisms, when electrons
are transferred from electron donors to
acceptors (Table 3.4). The greatest amount of
energy can be harvested by organisms by transferring
electrons to oxygen. However, under
anaerobic conditions, which commonly occur in
flooded soils with high organic matter contents
or in aquatic sediments, electrons must be
transferred to other electron acceptors; thus
progressively less energy is released with the
transfer to each of the following electron acceptors:
O2 > NO3 - > Mn 4+ > Fe 3+ >
SO4 2- > CO2 > H + (3.3)
As soil redox potential declines, the preferred
electron carriers are gradually consumed
(Table 3.4). As oxygen becomes depleted, for
example, the redox reaction that generates the
most energy is denitrification (transfer of electrons
to nitrate), followed by reduction of Mn 4+
to Mn 2+ , then reduction of Fe 3+ to Fe 2+ , then
reduction of SO4 2- to hydrogen sulfide (H2S),
then reduction of CO2 to methane (CH4). Thus
poorly aerated soils with high sulfate concentrations
(e.g., salt marshes) are less likely to
reduce CO2 to CH4 than are similar soils with
lower SO4 2- concentrations.
Many soil organisms carry out only one or a
few redox reactions, although certain bacteria
can couple the reduction of Mn 4+ and Fe 3+
directly to the oxidation of simple organic substrates
(Schlesinger 1997).Temporal and spatial
variations in soil redox potential alter the types
of redox reactions that occur primarily by altering
the competitive balance among these
organisms. Organisms that derive more energy
from their redox reactions (e.g., denitrifiers
compared to methane producers) will be competitively
superior, when they have an adequate
supply of electron acceptors.
Soil organic matter content is a critical component
of soils, affecting rates of weathering
and soil development, soil water-holding capacity,
soil structure, and nutrient retention. In
addition, soil organic matter provides the
energy and carbon base for heterotrophic soil
organisms (see Chapter 7) and is an important
reservoir of essential nutrients required for
plant growth (see Chapter 8). Soil organic
matter originates from dead plant, animal, and
microbial tissues, but includes a range of materials
from new, undecomposed plant tissues to
resynthesized humic substances that are thousands
of years old, whose origins are chemically
and physically unrecognizable (see Chapter 7).
Because soil organic matter is important to so
many soil properties, loss of soil organic matter