EXERCISE 21
Decomposition: Particulate
Organic Matter
Decomposition completes the biogeochemical
cycles that photosynthesis initiates. Thus, complete decomposition results in the conversion of
the organic (reduced) products of photosynthesis
back into the inorganic (generally oxidized)
constituents used as the reactants for photosynthesis (see Exercise 14). The major biogeochemical
cycles affected by decomposition are those of C,
N, P, S, and 0, although it is important to realize
that all of the minor constituents of biomass
(cations, trace metals, etc.) also are released
(mineralized) by decomposition. When plants or
animals senesce and die, both dissolved (DOM)
and particulate (POM) organic matter are available for degradation. Leakage of DOM from
dying cells and autolysis of the tissue increase
during senescence and reach maximum levels
soon after death. The DOM thus produced is
leached readily from the tissue into the aquatic
environment and can constitute as much as 30%
of the total amount of combined particulate and
dissolved organic matter in lake water. This
detrital DOM is available as an energy source for
microflora in the sediments and waters adjacent
to particulate detritus. The rate of degradation is
dependent on both the enzymatic capabilities of
the microflora and the environmental conditions
(see Exercise 20). Some compounds of the DOM
are more stable than others, but warmer temperatures and increased availability of oxygen
reduce their resistence to oxidation (refractility)
to some extent (Godshalk and Wetzel, 1978a).
Particulate detritus is colonized by various
microflora. The rate of degradation depends on:
(1) the composition (i.e., the refractility) of specific
substrates within the particles, (2) the ability of
microbes to get at the tissue (e.g., particle
size:surface area), and (3) the rate of microbial
metabolism as governed by enzymatic capacities,
temperature, and availability of electron acceptors (e.g., oxygen) and mineral nutrients
(Alexander, 1965; Godshalk and Wetzel, 1978b;
Webster and Benfield, 1986). A succession of
types of microflora associated with the detrital
particles occurs over time as a result of changes in
substrate availability and environmental conditions caused by their metabolism. Total microbial metabolism and biomass often increase
initially after colonization of the detritus as a
result of increased concentrations of organic
nitrogen relative to that of carbon. As the
resistance of the residual detritus increases with
continued decomposition, the degradability of
the detritus decreases. Organic nitrogen concentration then decreases relative to that of
carbon, resulting in high organic C:N ratios.
Measurement of the rates of decomposition of
organic matter in situ is difficult. Commonly,
changes in the dry weight of a known amount of
POM are measured over a period of time. Such
parameters as percent weight loss continually
change through time and only yield information
about the end result of decomposition. Decomposition is a continuous process, but the rate of
281
Decomposition: Particulate
Organic Matter
Decomposition completes the biogeochemical
cycles that photosynthesis initiates. Thus, complete decomposition results in the conversion of
the organic (reduced) products of photosynthesis
back into the inorganic (generally oxidized)
constituents used as the reactants for photosynthesis (see Exercise 14). The major biogeochemical
cycles affected by decomposition are those of C,
N, P, S, and 0, although it is important to realize
that all of the minor constituents of biomass
(cations, trace metals, etc.) also are released
(mineralized) by decomposition. When plants or
animals senesce and die, both dissolved (DOM)
and particulate (POM) organic matter are available for degradation. Leakage of DOM from
dying cells and autolysis of the tissue increase
during senescence and reach maximum levels
soon after death. The DOM thus produced is
leached readily from the tissue into the aquatic
environment and can constitute as much as 30%
of the total amount of combined particulate and
dissolved organic matter in lake water. This
detrital DOM is available as an energy source for
microflora in the sediments and waters adjacent
to particulate detritus. The rate of degradation is
dependent on both the enzymatic capabilities of
the microflora and the environmental conditions
(see Exercise 20). Some compounds of the DOM
are more stable than others, but warmer temperatures and increased availability of oxygen
reduce their resistence to oxidation (refractility)
to some extent (Godshalk and Wetzel, 1978a).
Particulate detritus is colonized by various
microflora. The rate of degradation depends on:
(1) the composition (i.e., the refractility) of specific
substrates within the particles, (2) the ability of
microbes to get at the tissue (e.g., particle
size:surface area), and (3) the rate of microbial
metabolism as governed by enzymatic capacities,
temperature, and availability of electron acceptors (e.g., oxygen) and mineral nutrients
(Alexander, 1965; Godshalk and Wetzel, 1978b;
Webster and Benfield, 1986). A succession of
types of microflora associated with the detrital
particles occurs over time as a result of changes in
substrate availability and environmental conditions caused by their metabolism. Total microbial metabolism and biomass often increase
initially after colonization of the detritus as a
result of increased concentrations of organic
nitrogen relative to that of carbon. As the
resistance of the residual detritus increases with
continued decomposition, the degradability of
the detritus decreases. Organic nitrogen concentration then decreases relative to that of
carbon, resulting in high organic C:N ratios.
Measurement of the rates of decomposition of
organic matter in situ is difficult. Commonly,
changes in the dry weight of a known amount of
POM are measured over a period of time. Such
parameters as percent weight loss continually
change through time and only yield information
about the end result of decomposition. Decomposition is a continuous process, but the rate of
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