Principles of terrestrial ecosystem ecology.pdf

152 7. Terrestrial Decomposition
Soil animals also mix the decomposing organic
matter into the soil. (3) Chemical alteration of
dead organic matter is primarily a consequence
of the activity of bacteria and fungi, although
some chemical reactions also occur spontaneously
in the soil without microbial mediation.
Dead plant material (litter) and animal
residues are gradually decomposed until their
original identity is no longer recognizable, at
which point they are considered soil organic
matter (SOM). Litter consists primarily of
compounds that are too large and insoluble to
pass through microbial membranes. Microbes
therefore secrete exoenzymes (extracellular
enzymes) into their environment to initiate
breakdown of litter. These exoenzymes convert
macromolecules into soluble products that can
be absorbed and metabolized by microbes.
Microbes also secrete products of metabolism,
such as CO2 and inorganic nitrogen, and
produce polysaccharides that enable them to
attach to soil particles.When microbes die, their
bodies become part of the organic substrate
available for decomposition.
The controls over organic matter breakdown
change radically once soil organic matter
becomes incorporated into mineral soil. The
soil moisture and thermal regimes of mineral
soil are quite different from those in the litter
layer. In the mineral soil, SOM can complex
with clay minerals or undergo nonenzymatic
chemical reactions to form more complex
compounds. Humus, for example, is a complex
mixture of chemical compounds with highly
irregular structure containing abundant aromatic
rings. Humus tends to accumulate in soils
because exoenzymes cannot easily degrade its
irregular structure (Oades 1989).
Decomposition is largely a consequence of
the feeding activity of soil animals (fragmentation)
and heterotrophic microbes (chemical
alteration). The evolutionary forces that shape
decomposition are those that maximize the
growth, survival, and reproduction of soil
organisms. Controls over decomposition are
therefore best understood in terms of the
controls over the activities of these organisms.
The ecosystem consequences of decomposition
are the mineralization of organic matter to
inorganic components (CO2, mineral nutrients,
and water) and the transformation of organic
matter into complex organic compounds that
are recalcitrant (i.e., resistant to further microbial
breakdown). In other words, decomposition
occurs to meet the energetic and
nutritional demands of decomposer organisms,
not as a community service for the carbon cycle.
Leaching of Litter
Leaching is the rate-determining step for mass
loss of litter when it first falls to the ground.
Leaching is the physical process by which
mineral ions and small water-soluble organic
compounds dissolve in water and move through
the soil. During leaf senescence, many of the
compounds in a leaf are broken down and
transported to other plant parts (see Chapter
8).This resorption process is still actively occurring
when the leaf is shed, so the senesced leaf
contains relatively high concentrations of
water-soluble breakdown products that are
readily leached. Leaching begins when tissues
are still alive and is most important during
tissue senescence and when litter first falls to
the ground. Leaching losses from litter are proportionally
more important for nutrients than
for carbon. Leaching losses from fresh litter are
greatest in environments with high rainfall and
are negligible in dry environments. Compounds
leached from leaves include sugars, amino
acids, and other compounds that are labile
(readily broken down) or are absorbed intact
by soil microbes. Leachates frequently support
a pulse of microbial growth and respiration
during periods of high litterfall.
Litter Fragmentation
Fragmentation creates fresh surfaces for microbial
colonization and increases the proportion
of the litter mass that is accessible to microbial
attack. Fresh detritus is initially covered by a
protective layer of cuticle or bark on plants or
of skin or exoskeleton on animals. These outer
coatings are designed, in part, to protect tissues
from microbial attack. Within plant tissues, the
labile cell contents are further protected from