Principles of terrestrial ecosystem ecology.pdf

166 7. Terrestrial Decomposition
lignin:N ratio, are frequently used as predictors
of decomposition rate.
The effects of litter quality on decomposition
rate often depend on the age of the litter. Highquality
litter, for example, loses its labile carbon
so quickly that the remaining old litter may
have a lower decomposition potential than
litter that initially had a low litter quality and
slow decomposition rate (Berg and Ekbohm
1991).
The availability of belowground resources is
the major ecological control over litter quality.
Rapidly growing plants from high-resource
sites typically produce litter that decomposes
quickly because the same morphological and
chemical traits that promote NPP also regulate
decomposition (Hobbie 1992). Both NPP and
decomposition are enhanced by a high allocation
to leaves and by the production of leaves
with a short life span. These tissues decompose
rapidly because they have high concentrations
of labile compounds such as proteins and
low concentrations of recalcitrant cell-wall
components such as lignin (Reich et al.
1997). Consequently, species from productive
sites produce litter that decomposes rapidly
(Cornelissen 1996) (Fig. 7.9). Species differences
in litter quality make up an important
mechanism by which plant species affect
ecosystem processes (see Chapter 12) (Hobbie
1992) and are excellent predictors of landscape
patterns of litter decomposition (Flanagan and
Van Cleve 1983).
Soil Organic Matter
Both the age and the initial quality of SOM
influence its rate of decomposition. As litter
decomposes, its decomposition rate declines,
because microbes first consume the more labile
substrates, leaving progressively more recalcitrant
compounds in the remaining litter (Fig.
7.2). Through fragmentation by soil invertebrates
and these chemical alterations, the litter
becomes converted to soil organic matter. As
microbes die, chitin and other recalcitrant components
in their cell walls comprise an increasing
proportion of the litter mass (actually litter
plus microbial mass), and nonenzymatic reactions
produce recalcitrant humic compounds.
All these processes contribute to a gradual
reduction in organic matter quality as SOM
ages. The C:N ratio also declines as decomposition
proceeds, because carbon is respired
away, and some of the mineralized nitrogen is
incorporated into humus. The decline in C:N
ratio is not, however, an indicator of increased
nitrogen availability, because the nitrogen
becomes incorporated into aromatic rings and
other chemical structures that are recalcitrant.
In summary, in SOM, as in litter, the carbon
quality is a better predictor of decomposition
rate than is the C:N ratio or the nitrogen
concentration of SOM (Berg and Staaf 1980,
Melillo et al. 1982).
Site differences in nutrient availability influence
SOM decomposition primarily through
their effects on the carbon quality of litter and
SOM, rather than through direct nutrient
effects on SOM decomposition. Sites with high
productivity and litter quality typically produce
a low-lignin SOM that decomposes readily
(Van Cleve et al. 1983). As in the case of fresh
litter, SOM decomposition rate does not show
a consistent response to nutrient addition
(Haynes 1986, Fog 1988), suggesting that nutrients
seldom directly regulate SOM decomposition.
Decomposition of SOM increases in
response to nitrogen addition primarily when
the organic matter consists of labile carbon substrates,
for example when straw is plowed into
agricultural soils (Mary et al. 1996) or when
root exudation is enhanced by elevated CO2
(Hu et al. 2001) (see Chapter 9).
The heterogeneous nature of SOM makes it
difficult to identify the chemical controls over
its decomposition. It is a mixture of organic
compounds of different ages and chemical compositions.
Components of SOM include fragments
of recently shed root and leaf litter,
together with soil organic matter that is thousands
of years old (Oades 1989). These different
aged components of SOM can be separated
by density centrifugation, because recently produced
particles are less dense than older ones
and are less likely to be bound to mineral particles.
Soils in which a large proportion of the
SOM is in the light fraction generally have
higher decomposition rates (Robertson and
Paul 2000).Alternatively, soil can be chemically