Environmental Engineering Reference
In-Depth Information
A wide array of macroinvertebrates (e.g., polychaetes and aquatic insects) rely on con-
sumption of nonliving detritus in sediments or organic accumulations. Abundance or pro-
duction of animals is often correlated with inputs of detritus (see Figure 3.6 in Chapter 3).
By inference, this partially decomposed organic matter serves as a food resource although
the exact pathways (microbial mediation or not) probably vary among organisms and
among ecosystems. Microbial mediation occurs through at least two mechanisms: In some
cases the microbial biomass associated with detritus is the predominant contributor to
assimilable carbon (see Leal et al. 2011), or the action of microbial enzymes makes the non-
living detrital substrate more digestible by consumers. Whether microbes are involved
directly, indirectly, or not at all, the transfer of detritus to higher trophic levels is an
important path of energy flow in almost all ecosystems (see Chapter 3;
Cebrian 1999
).
The process of mineralizing the organic compounds in decaying organic matter returns
many elements to a chemical form available for uptake by autotrophs or heterotrophic
microbes and enhances transport to other ecosystems (see Chapter 7). For most ecosystems
the pool of limiting nutrients (nitrogen and phosphorus) contained in organic matter is much
greater than the pool of inorganic forms and so ecosystem productivity may be controlled by
rates of decomposition. The fact that much of the particulate plant litter entering aquatic eco-
systems has an overabundance of carbon relative to nitrogen, phosphorus, and such (with
respect to the needs of consumers; (Box 3.1,
Figure 4.8
) implies that microbes metabolizing
the organic carbon face a shortage of other elements unless external supplies are available.
Most heterotrophic microorganisms have very efficient systems for acquiring external nutri-
ents, and so as they grow there is a significant potential for immobilization of nutrients from
the medium and incorporation in the detritus-microbe complex for some period of time.
This chapter will describe general processes of decomposition. Most of the basic concepts,
processes, and even methodology are either similar or parallel for aquatic and terrestrial
decomposition although certain aspects, metabolism of dissolved organic matter in particular,
have received much more attention in aquatic ecosystems. The basic principles of organic matter
decay do not differ greatly between aquatic and terrestrial ecosystems and, for instance, the
importance of litter quality (lignin, nitrogen, etc.) has been demonstrated numerous times for
both classes of ecosystems. There are, however, some substantial differences, the most obvious
of which is the general importance of moisture availability in regulating organic matter decay
and metabolic activity in many terrestrial systems (
Sanderman and Amundson 2005
).
Less obvious is the much greater opportunity for secondary reactions of decomposition
products in soils than aquatic systems. The process of soil formation (humification) is key
to many properties of terrestrial systems, and these processes can occur because removal
of decomposition by-products is essentially zero for most soils. In many (but not all)
aquatic systems, decomposition is balanced against physical transport (loss) and so there
is often inadequate time for secondary recombination of decomposition by-products into
new organic compounds. Additionally, incomplete decomposition within either flowing
water or connected aquatic ecosystems allows for substantial cross-boundary fluxes of
organic matter. The organic content of agricultural soils is a key control on their fertility
and many agricultural practices such as plowing or adding manures are specifically
intended to affect the balance between organic input and decomposition (
Hendrix et al.
1986
). Nutrient availability in agricultural soils is affected by rates of release from organic
matter as well as immobilization of inorganic forms in microbial biomass growing on
