Environmental Engineering Reference
In-Depth Information
FIGURE 4.3
Hypothetical mass loss curve showing
processes likely to be important during various phases.
Leaching
Bulk POM decay
Recalcitrant
Time
present, so the actual quantity of mass disappearing over sequential time periods will
get smaller as the amount of remaining material declines. The exponential constant (generally
denoted as
k
with units of time
2
1
) is a simple measure of differences in rates across types
of litter, temperatures, presence/absence of certain organisms. and so on. In an extensive
literature review,
Enriquez et al. (1993)
reported median
k
values for various plant groups
ranging across three orders of magnitude. Slowest were perennial leaves from broadleaf trees
(0.001/day), intermediate were seagrasses and macroalgae (0.01/day), and fastest were
microalgae (nearly 0.1/day). More complex models of mass loss are justified in some cases
(
Findlay et al. 1996
), but the power of comparison to other observations argues for the simple
and common model in most cases.
The measurement of disappearance of mass from a litter bag is a very coarse (or inte-
grated) view of what is actually happening to the original organic matter. In fact, the
actual pathway of disappearance has large implications for how that original organic mat-
ter might contribute to secondary production, be transported to other ecosystems, or inter-
act with other element cycles. The predominance of any given pathway varies greatly
among ecosystems and types of material but in general mass will be lost as soluble com-
pounds are released (
Figure 4.4
), smaller particles are shed, and gases containing carbon,
primarily CO
2
, are released. Some of these products of mass loss (dissolved organic mat-
ter, DOM; fine particulate organic matter, FPOM) are fully capable of serving as a food
resource for other organisms while the mineralization of organic carbon to CO
2
represents
a dead end in terms of chemical potential energy. The partitioning between loss to other
organic forms and CO
2
determines what organisms can derive energy from a unit input of
POC (particulate organic matter). The relative magnitude of the various pathways depends
on multiple factors, most of which have been fairly well studied.
Loss of particles depends on at least three things: fragility of the original material, phys-
ical stresses in the system, and perhaps most importantly the prevalence of animals capa-
ble of physically breaking litter into smaller pieces. Plant litters with a relatively low
quantity of structural material are more susceptible to physical fragmentation, and for
some fragile aquatic plants these losses can be rapid even without the activity of leaf-
shredding invertebrates (
Wallace and O'Hop 1985
).
Loss of soluble organic compounds generally dominates the very early, rapid mass loss
phase of decomposition soon after litter enters the water or is subject to precipitation/
flooding in terrestrial systems. This part of mass loss appears as a very rapid decline over
