Environmental Engineering Reference
In-Depth Information
TABLE 4.1
Example of range in lignin, nitrogen, and decay rates (k) for several common types of leaf litter.
(d
2
1
)
Species
Lignin (%)
SEM of Lignin
Mean N (%)
SEM of N
Mean
SEM of
k
k
Elm
11.1
0.4
1.1
0.04
2
0.026
0.006
Ash
13.9
2.8
0.73
0.02
2
0.017
0.003
Alder
18.6
0.3
1.91
0.08
2
0.01
0.002
Maple
14.3
0.9
1.1
0.13
2
0.01
0.001
Willow
19.7
0.3
1.28
0.13
2
0.01
0.001
Hazel
23
3
1.15
0.06
2
0.008
0.0004
Sycamore
34.3
1.7
0.8
0.07
0.008
0.0006
2
Oak
23.9
0.5
0.45
0.03
0.005
0.0004
2
Beech
35.7
0.8
1
0.05
0.003
0.0002
2
Values are from
Schindler and Gessner (2009)
, who report a significant relationship between percent of lignin and the decay rate in coarse
mesh litter bags. SEM
5
standard error of the mean.
(
Table 4.1
). There are several reasons these macromolecules have significant effects on
mass loss. Most obvious is simply their physical nature; compounds that play a role in
supporting plant tissues must resist decay. These structural biomolecules can have com-
plex physical structure (lignocellulose), making it difficult for enzymes to reach particular
bonds. Second, the fact that they are polymeric and often heterogeneous means that
microbes must release extracellular enzymes to break bonds (via hydrolysis or oxidation)
to liberate the simpler subunits that they can break down. Moreover, some subunits (phe-
nolic rings) are partially oxidized and so the energy yield is less than for a carbohydrate.
The other important class of materials affecting decay rates includes the nutrients
(mainly but not exclusively nitrogen and phosphorus) that typically are underrepresented
in plant litter relative to the needs of micro- or macroconsumers (Chapter 2, Box 3.1). To
produce new biomass microbes require about 1 atom of nitrogen for about every 10 atoms
of carbon. Accounting for the necessary loss of carbon to respiration means that organic
detritus with less than 1 atom of nitrogen per 20 atoms of carbon will be poor in nitrogen
relative to microbial demands (
Figure 4.8
). The relative paucity of nutrient can affect decay
in two counteracting ways; slowing decomposition of low-nutrient litter due to the low
biomass of decomposers, or increasing decomposition to compensate for low nutrient
availability. In the first scenario the ultimate population size of decomposers and therefore
rate of mass loss is limited by the quantity of nitrogen or phosphorus. In the alternative
case, animals and/or microbes increase consumption to compensate for lower nutritive
value and overall mass loss is greater. Both these cases assume the organic carbon com-
pounds themselves do not limit decay.
Aside from the major structural components of plant material or the content of limiting
nutrients (nitrogen, phosphorus), certain plant secondary compounds have been predicted
to have strong effects on both decay rates and nitrogen cycling. Plant secondary com-
pounds are not an essential part of plant metabolism but serve specific functions, often as
