Environmental Engineering Reference
In-Depth Information
FIGURE 4.4
Illustration of the various pathways
of organic matter decomposition and opportunities for
immobilization of other nutrients (N, P) during micro-
bial growth. Any of the components on the right side
(DOC, nutrients) may be transported and used in
other ecosystems. For many (but certainly not all)
aquatic systems the input of detritus may be an export
from another ecosystem.
Detritus Decomposition
Dead plants
DOC
Fungi
CO
2
CH
4
Bacteria
Fine
POM
N, P, etc.
External
nutrients
the first few days followed by slower rates and a flattening of the curve. Quantities of
release as well as constituents vary dramatically among litter types with mass loss in the
first week or so ranging from negligible to more than 20%. These compounds are obvi-
ously available for subsequent metabolism by heterotrophic microbes and this can be a
large, although short-term, subsidy.
Dissolved organic matter released from litter in aquatic systems or leached from soils
can support food webs, interact with inorganic nutrient pools, alter light regimes, and can
complex metals and toxic organic compounds in diverse ways (
Findlay and Sinsabaugh
2003
). Many of the principles and questions relevant to the study of POC decay apply
equally to DOC although the likelihood of transport is much greater and protozoa and
invertebrates play a much smaller role in DOC degradation.
The ultimate fate of POC is mineralization to CO
2
, which, as mentioned earlier, is of no
further energetic use to heterotrophs and often leaves the aquatic ecosystem. Generation
of CH
4
(or other volatile organic compounds of low molecular weight) is susceptible to
oxidation by the appropriate microbes (e.g., methane oxidizers) and so may contribute fur-
ther to secondary production.
Measurement of mass loss from litter bags has been an informative, comparative, and
integrative approach to studying decomposition (see
Benfield 2006
). However, the inability
to identify which process is responsible for how much of the mass loss reduces the utility
of litter bags in appreciating the subsequent uses of detrital organic matter and conse-
quences for cycling of other elements. This technique also does not allow for study of
longer-term decay processes since gases, small particles, and soluble compounds simply
disappear from the litter bag. Decay of these components is often studied under artificial
conditions of containment (such as bioassays of DOC consumption) or by integrative mea-
sures such as net CO
2
released from a fine sediment or soil. These latter approaches are
nonspecific since CO
2
can derive from a range of OM present while the litter bag approach
essentially follows a “cohort” of litter through early stages of decay. Measurements of spe-
cific processes such as CO
2
release or nitrogen immobilization during a litter bag study
add greatly to understanding the macrodecomposition of POM.
