Environmental Engineering Reference
In-Depth Information
A diverse array of extrinsic factors affects decay rates. In terrestrial ecosystems, the
most important are temperature (
Figure 4.9
) and moisture. Many studies of decay have
shown an increase in decay at warmer temperatures (
e.g., Acu
˜
a et al. 2008
) and recently
it was demonstrated that diel variability in temperature could also affect mass loss (
Dang
et al. 2009
). Despite the documented importance of temperature as a regulator of decom-
position, some organisms carry out degradation even at fairly low temperatures. The best
example may be the aquatic hyphomycete fungi, found in cold-water streams of which the
temperature optimum is generally
10
C, allowing them to metabolize leaf litter in
,
streams during winter.
Because most plant litters are poor in inorganic nutrients (nitrogen, phosphorus) rela-
tive to decomposer biomass, it is not surprising that environments with some external
nutrient supply have more rapid decomposition. If the organic carbon in litter is available,
then external nutrients allow for higher biomass of decomposers with resultant increases
in carbon assimilation and hence mass loss (
Suberkropp et al. 2010
). Surface waters in par-
ticular can provide a nutrient subsidy that enhances detritus decay.
Suberkropp and
Chauvet (1995)
showed a six-fold increase in mass loss of a common litter type across a
range of ambient nitrogen concentrations in a series of streams. Similarly,
Robinson and
Gessner (2000)
showed an increase in mass loss in leaf packs amended with slow-release
phosphorus fertilizer. The stimulatory effects of external nutrients can be due to various
processes. For instance, if inorganic nutrients are available in the medium, microbes will
not necessarily elaborate extracellular enzymes to acquire nutrients from organic materials.
This may allow reallocation to enzymes targeting organic carbon compounds and faster
litter mass loss.
For aquatic ecosystems environmental pH is thought to constrain decomposer activity
either through effects on microbes or by restricting abundances of leaf-shredding macroin-
vertebrates. Streams receiving acid mine drainage often show lower decay rates (see
Niyogi et al. 2009
). Acidic bogs can have pH as low as 2 or 3 (
Mitsch and Gosselink 1993
),
which in combination with low oxygen availability leads to very slow decay. Often these
bogs, which by definition get all their water from precipitation, also have low concentra-
tions of inorganic nutrients and base cations so the direct pH effect may be hard to
5
k
= 0.0376
MAT
+ 0.0819
R
2
= 0.2883
P
<0.001
4
3
2
1
0
-10
0
10
20
30
Mean annual temperature (
°
C)
FIGURE 4.9
The effect of mean annual temperature (MAT) on decomposition rates (
Zhang et al. 2008
).
