Environmental Engineering Reference
In-Depth Information
e
-
CO
2
CH
2
O
Production
Inorganic
Organic
Decomposition
CH
2
O
CO
2
e
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Ecosystem boundary
FIGURE 4.1
Diagram illustrating conversion of oxidized carbon to the organic form (requires energy) and the
back-reaction, which releases energy. Similar reactions occur for all the essential elements, which must be “fixed”
into the organic form and may release energy upon breaking the organic bonds.
more available for assimilation by plants. Thus, decomposition completes the cycle begun
with carbon fixation by releasing energy and elements to support other organisms.
In the absence of complete decomposition to CO
2
and inorganic nutrients, organic mat-
ter would accumulate and essentially sequester elements ultimately causing the ecosystem
to wind down due to depletion of limiting elements. So decomposition is essential to
maintain primary production, nutrient cycling, and other ecosystem processes. Other
products of decomposition such as fine particles and dissolved organic matter allow for
transport of organic carbon from locations of high productivity or large allochthonous
loadings to downstream portions of flowing waters, deeper areas of lakes, or parts of estu-
arine and coastal systems connected by tidal exchange. Thus, decomposition in one loca-
tion can subsidize energy flow in another. Organic matter sequestration in soils and
sediments occurs to varying degrees in all ecosystems and has obviously occurred to a
much greater degree in the geologic past resulting in large stockpiles of fossil fuels.
Potential for future conversion of atmospheric CO
2
to longer-term storage is a critical fac-
tor in understanding how the global climate will change over the next decades (Chapters 6
and 17).
Most aquatic ecosystems are net heterotrophic (definition provided in Chapter 2; see,
e.g.,
Teodoru et al. 2009
), suggesting that they decompose large amounts of allochthonous
material. Particulate plant litter is a major input to many ecosystems (leaves in headwater
streams, plant litter in wetlands) and since local P/R is often less than 1 the portion of
these inputs that is metabolized rather than stored or exported must be large relative to
NPP. Understanding how decomposition proceeds (what are controls, which organisms
are responsible) adds to knowledge of overall ecosystem function. Studies of particulate
organic matter (POM) decay have logically been dominated by examination of plant litter
since this is the main source of particulate detritus to most ecosystems. However, the deg-
radation of animal and microbial biomass is a major term in subsequent carbon flow (see
Chapter 3) and may represent an important pathway of nutrient transport (
Bartz and
Naiman 2005
). For instance, the movement of anadromous salmon from the sea to small
streams represents an important flux of nitrogen and the influence of this source can be
seen in the terrestrial system surrounding the spawning stream (
Naiman et al. 2002
).
