Environmental Engineering Reference
In-Depth Information
CHAPTER
4
Organi
c Matter Decomp
osition
Stuart E.G. Findlay
Cary Institute of Ecosystem Studies, Millbrook, NewYork
INTRODUCTION
Decomposition
is used in two distinct senses in ecosystem science. At the macroscale,
decomposition is the conversion of organic matter from large and recognizable physical
forms to small particles, soluble compounds, and gases. At the molecular scale, elements
are generally converted from complex organic molecules such as carbohydrates and pro-
teins to simpler forms such as individual sugars, amino acids, and eventually to inorganic
compounds such as ammonium and carbon dioxide. As these compounds are released,
the chemical energy tied up in the organic bonds is available for energy demands of the
organisms carrying out the decomposition.
It is important to distinguish the two scales of decomposition. In the macro view, materi-
als “lost” from the recognizable parent material may well be useable as food resources by
other organisms. Release of fine particles and soluble organic compounds and even reduced
gases (methane) may represent significant resources for other portions of the ecosystem.
Additionally, these compounds may be transported to other ecosystems where they may
well represent a significant subsidy. For example, terrestrial inputs to lakes via leaf-fall, or
organic matter carried by streams, may fuel the food web and appear as a major portion of
the organic content of higher trophic levels (e.g.,
Pace et al. 2004
). Simple sugars, nucleic
acids, or amino acids released during macroscale decomposition can be easily incorporated
by microbes and probably are removed quickly before there is much opportunity for trans-
port. Ultimately, as bonds are cleaved, organic carbon (and its chemical potential energy) is
released and may be oxidized to CO
2
, a form with no further energetic value for consumers.
In the simplest example, the release of CO
2
from organic matter is the reverse of carbon fixa-
tion (primary production) (
Figure 4.1
). Breaking of the bonds formed during carbon fixation
provides that “fixed” energy to heterotrophs. Parallel with release of carbon is regeneration
of other elements held in organic matter (e.g. nitrogen, phosphorus, sulfur) to simpler forms
