Principles of terrestrial ecosystem ecology.pdf

in estimating decomposition at the ecosystem
scale (Box 7.1). Both of these approaches
indicate that stand-level decomposition rate
depends not only on environment, as discussed
earlier, but also on the amount and quality of
recent carbon inputs to soils (Fig. 7.14). The
quantity of carbon input to soils, in turn, generally
depends on NPP. Carbon quality is also
highest in productive stands.
Since GPP and NPP are important determinants
of stand-level decomposition rate, it is not
surprising that the controls over stand-level
decomposition are similar to those for GPP and
NPP. In other words, decomposition is ultimately
controlled by the availability of soil
resources, disturbance regime, and climate (Fig.
7.14). Measurements of soil respiration, which
includes both heterotrophic and root respiration,
are consistent with this generalization. Soil
The quantity of soil carbon differs dramatically
among ecosystems (Post et al. 1982).
The total quantity of carbon in an ecosystem,
however, gives relatively little insight into
its dynamics. Tropical forests and tundra,
for example, have similar quantities of
soil carbon, despite their radically different
climates and productivities. The simplest
measure of soil carbon turnover is its
residence time estimated from the pool size
and carbon inputs (Eq. 7.3). These measurements
show that, even though tropical
forests and arctic tundra have similar size
soil carbon pools, the turnover may be 500
times more rapid in the tropical forest. More
sophisticated approaches to estimating
soil carbon turnover using carbon isotopes
(Ehleringer et al. 2000) lead to a similar
conclusion. In the tropics, 85% of the 14 C
that entered ecosystems during the era of
nuclear testing in the 1960s has been converted
to humus, whereas this proportion is
only 50% in temperate soils and approximately
0% in boreal soils (Trumbore 1993,
Trumbore and Harden 1997). This comparison
clearly indicates more rapid turnover of
Box 7.1. Isotopes and Soil Carbon Turnover
Decomposition at the Ecosystem Scale 171
respiration correlates closely with NPP (Raich
and Schlesinger 1992) (Fig. 7.15). Carbon loss
through soil respiration is about 25% higher
than carbon inputs through NPP, suggesting
that about 25% of soil respiration derives from
roots, and the rest comes from decomposition
(Raich and Schlesinger 1992). Both NPP and
decomposition are higher in the tropics than in
the arctic and higher in rain forests than in
deserts, due to similar environmental sensitivities
of plants and decomposers. Likewise, plant
species that are highly productive produce litter
of higher quality than do species of low potential
productivity. Habitats dominated by productive
species are therefore characterized by
high rates of litter decomposition (Hobbie
1992), high concentrations of labile carbon, and
high microbial biomass (Zak et al. 1994), all
contributing to the high stand-level decomposi-
soil organic matter in the tropics than at high
latitudes.
Carbon isotopes can also be used to estimate
the impacts of land use change on
carbon turnover in situations in which the
vegetation change is associated with a change
in carbon isotopes. In Hawaii, for example,
replacement of C3 forests by pastures dominated
by C4 grasses causes a gradual change
in the carbon isotope ratio of soil organic
matter from values similar to C3 plants
toward values similar to C 4 plants (Townsand
et al. 1995). This information can be used to
estimate the quantity of the original forest
carbon that remains in the ecosystem:
%C
S1
C - C
=
C - C
S2 V2
V1 V2
¥
100
(B7.1)
where %CS1 is the percentage of soil derived
from the initial ecosystem type, CS2 is the 13 C
content of soil from the second soil type, CV2
is the 13 C content of soil from the second
vegetation type, and CV1 is the 13 C content of
vegetation from the initial ecosystem type.