Principles of terrestrial ecosystem ecology.pdf

separated into distinct fractions, such as watersoluble
compounds, humic acids, and fulvic
acids, that differ in average age and ease of
breakdown. SOM as a whole typically has a residence
time of 20 to 50 years, although this can
range from 1 to 2 years in cultivated fields to
thousands of years in environments with slow
decomposition rates. Even in a single soil, different
chemical fractions of SOM have residence
times ranging from days to thousands of
years. Computer simulations of decomposition
rate capture ecosystem carbon dynamics more
effectively when they distinguish among these
different soil carbon pools (Parton et al. 1993,
Clein et al. 2000).
Decomposition in the rhizosphere is more
rapid than in bulk soil for reasons that are
poorly understood. The rhizosphere makes up
virtually all the soil in fine-rooted grasslands,
where the average distance between roots is
about 1mm, whereas forests are less densely
rooted (often 10mm between roots) (Newman
1985). Roots alter the chemistry of the rhizosphere
by secreting carbohydrates and absorbing
nutrients.These processes are most active in
the region behind the tips of actively growing
roots (Fig. 7.11) (Jaeger et al. 1999b). The
growth of bacteria in the zone of exudation
(Norton and Firestone 1991) is supported by
abundant carbon availability (20 to 40% of
NPP; see Table 6.2) and is therefore limited
most strongly by nutrients (Cheng et al. 1996).
Bacteria must acquire their nutrients for
growth by breaking down SOM. In other words,
plant roots use carbon-rich exudates to “prime”
the decomposition process in the rhizosphere,
just as you might use water to prime a pump.
Microbial immobilization of nutrients in the
rhizosphere benefits the plant only if these
nutrients are subsequently released and
become available to the root. Two processes
may contribute to the release of nutrients from
rhizosphere microbes: First, protozoa and
nematodes may graze the populations of
rhizosphere bacteria, using bacterial carbon to
support their high energetic demands and
excreting the excess nutrients (Clarholm 1985).
Second, as the root matures and exudation rate
declines, those bacteria that survive predation
may become energy limited and break down
Factors Controlling Decomposition 167
Microbial
processes
Predation by
protozoans
Rapid
bacterial
growth
Bacterial
starvation
SOM breakdown
N min.
N excr.
N immob.
ROOT
Root
processes
Nitrogen
uptake
Root
exudation
Sloughing of
root cap
Figure 7.11. Root and microbial processes
in the rhizosphere and the resulting effects on soil
organic matter breakdown and nitrogen dynamics in
the rhizosphere. N immob., N immobilization by
rhizosphere microbes; N excr., N excretion by
protozoa; N min., N miniralization by carbon-starved
microbes.
nitrogen-containing compounds to meet their
energy demands, releasing the nitrogen into the
rhizosphere as ammonium. The relative contribution
of grazing and starvation in these
processes is unknown, but net nitrogen mineralization
in the rhizosphere has been estimated
to be 30% higher than in bulk soil. Rhizosphere
decomposition occurs most readily in soils with
relatively labile soil carbon and low soil lignin
(Bradley and Fyles 1996) and therefore may
occur to a greater extent in grasslands or early
successional communities than in mature
forests. Rhizosphere decomposition may be
more sensitive to factors influencing plant carbohydrate
status (e.g., light and grazing) than
to soil environment (Craine et al. 1999), so the
nature of controls over decomposition (soil
environment vs. plant carbohydrate status)
could differ substantially among ecosystems.
However, the extent of rhizosphere decomposition
and the nature of its ecological controls
are not well characterized under field conditions,
so it is difficult to evaluate its ecological
importance.
Mycorrhizal fungi are functionally an
extension of the root system, allowing the
root–fungal symbiosis to absorb nutrients at a
distance from the root. The mycorrhizosphere