Principles of terrestrial ecosystem ecology.pdf

acids are metabolized more readily than cellulose
and proteins, respectively. (2) Some chemical
bonds are easier to break than others. Ester
linkages that bind phosphate to organic skeletons
and peptide bonds that link amino acids to
form proteins, for example, are easier to break
than the double bonds of aromatic rings. For
these reasons, the nitrogen in proteins is much
more available to microbes than the nitrogen
contained in aromatic rings. (3) Compounds
like lignin that have a highly irregular structure
do not fit the active sites of most enzymes,
so they are broken down much more slowly
than are compounds like cellulose, which
consist of chains of regularly repeating glucose
units. (4) Some soluble compounds such as phenolics
and alkaloids are toxic and kill or reduce
the activity of microbes that absorb them. (5)
Organic nitrogen and phosphorus are the major
sources of nutrients for supporting microbial
growth, so organic matter, such as straw,
that contains low concentrations of these elements
may not provide sufficient nutrients to
allow microbes to use fully the carbon present
in the litter.
All of these chemical properties influence
decomposition, but their relative importance is
not well understood. Nonetheless, any of these
properties can serve as a predictor of decomposition
rate because the properties tend to be
strongly correlated with one another. The ratio
of carbon concentration to nitrogen concentration
(C:N ratio), for example, has frequently
been used as an index of litter quality, because
litter with a low C:N ratio (high nitrogen
concentration) generally decomposes quickly
(Enríquez et al. 1993, Gholz et al. 2000).
However, neither the nitrogen concentration
of the litter nor the nitrogen availability in the
soil directly influences the decomposition rate
in most natural ecosystems (Haynes 1986,
Prescott 1995, Prescott et al. 1999, Hobbie and
Vitousek 2000); this suggests that C:N ratio is
not the chemical property that directly controls
decomposition in these ecosystems. This contrasts
with agricultural residues such as straw,
which have a low nitrogen concentration and a
high concentration of moderately labile carbon
sources like cellulose and hemicellulose. Nitro-
Factors Controlling Decomposition 165
gen concentration appears to limit directly the
decomposition rate of organic matter primarily
when labile carbon substrates are available to
support microbial growth (Haynes 1986). This
is more likely to occur in the rhizosphere than
in fresh litter. Under other circumstances,
carbon lability rather than nitrogen may be
the primary control over decomposition rate
(Hobbie 2000). Despite our uncertainty of the
mechanistic role of C:N ratio in decomposition,
many biogeochemical models use this
ratio as a predictor of decomposition rate when
different ecosystem types are compared (see
Chapter 9).
In recalcitrant litter, the concentration of
lignin or the lignin:N ratio is often a good predictor
of decomposition rate (Berg and Staaf
1980, Melillo et al. 1982, Taylor et al. 1989)
(Fig. 7.10), again suggesting an important role
of carbon quality in determining decomposition
rates of litter. The carbon quality of litter is
probably best defined in terms of the classes of
organic compounds present and the enzymatic
potential of the decomposer community, as
described later.This information is available for
so few ecosystems, however, that more readily
measured properties, such as C:N ratio or
Decomposition constant
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
10 20 30 40 50 60 70 80 90
Initial lignin:nitrogen ratio
Figure 7.10. Relationship between the decomposition
constant and the lignin : N ratio of litter.
(Redrawn with permission from Ecology; Mellillo et
al. 1982.)