7. Decomposition and Soil Organic Matter Dynamics
use a two-pool constrained model once the size of
the resistant pool is known (Paul et al. 2000).
Statistical analysis of the curves is most easily
accomplished when the CO2 evolution data are expressed on a rate per unit time basis rather than as
cumulative curves (Hess and Schmidt 1995). Use
of all the data in a single regression equation with
separate parameter estimates for each treatment
provides much greater statistical power than analyzing each treatment in a separate regression (Willson et al. in press). The effect of possible autocorrelation from repeated measurements of the same
sample also can be statistically corrected (Paul et
al. in press).
Use of Tracers
The isotopes of C include the radioactive 14C occurring in minute quantities in the atmosphere and
13C, which has a stable natural abundance of 1.1 %.
The 14C02 produced in the atmosphere by the interaction of cosmic radiation with N2 is incorporated into plants during photosynthesis at a fairly
constant rate, and can be measured with carbon dating techniques (Goh 1991). This is expensive and
not readily accessible, but together with acid hydrolysis gives the best estimates of the size and
turnover of the old resistant SOM fraction (Paul et
al. 1998; Leavitt et al. 1997). Added 14C, in the
form of uniformly labeled plant residues exposed
to a high 14C atmosphere, give exceptionally useful
information on organic matter breakdown and the
incorporation of C into microbial biomass (Wolf et
al. 1994). It is difficult to produce uniformly labeled
substrates, but field experiments can be undertaken
with commercially available, specifically labeled
substrates if adequate health and safety precautions
for working with a radioactive isotope are followed
(SchimeI1993).
The stable isotope 13C has been utilized in experiments in which plants have been 13C-Iabeled in
the laboratory in a fashion similar to that used for
radioactive elements. Reliable automated mass
spectrometers to measure the stable isotopes are
available in many laboratories. The naturally occurring discrimination against 13C02 by plants with
a C 3 photosynthetic pathway relative to those with
a C4 pathway provides a powerful, naturally occurring label in many parts of the world. This requires
a C r C 4 plant growth switch, such as the growth of
113
C 4 com or sorghum on sites previously forested
with C 3 trees, or the growth of C 3 wheat, rice, cold
season grasses, or most trees on prairie or savanna
soils that previously had C4 native vegetation. Sites
that have a mixed signal make it possible to grow
either a C 3 or C 4 test species. The 13C either enriched or depleted in the test species can be used to
measure the fate of test species plant residuesboth their incorporation into various soil biological,
chemical, and physical fractions as well as changes
in their pool sizes and flux rates.
13C studies are particularly useful when combined with CO 2 evolution studies (Paul et al. 1999).
The measurement of the 13C02 evolved during extended laboratory incubation yields estimates of the
active and slow pool sizes and decomposition kinetics for the labeled material. Details of these techniques are available in Schimel (1993), Boutton
(1996), Coleman and Fry (1991), and Paul et al.
(1998, 1999). The element N is closely associated
with soil C, and the stable isotope 15N has been
utilized in many residue decomposition and SOM
turnover studies. It, too, can provide much important information on decomposition as well as on the
microbial growth and N immobilization that occurs
during decomposition (Hauck et al. 1994).
Acknowledgments We thank a number of individuals for providing helpful discussion and insights
during the preparation of this chapter. We thank in
particular discussants at the 1996 LTER Soil Methods Standardization Workshop held in Sevilleta,
New Mexico, and especially M.E. Harmon, 1.M.
Blair, P. Sollins, E.T. Elliott, and D. Harris. This
work was supported by the National Science Foundation LTER program and the Michigan Agricultural Experiment Station.
References
Aber, J.D.; Melillo, J.M.; McClaugherty, C.A. Predicting
long-term patterns of mass loss, nitrogen dynamics,
and soil organic matter formation from initial fine litter
chemistry in temperate forest ecosystems. Can. 1. Botany 68:2201-2208; 1990.
Anderson, T.; Domsch, K.H. Ratios of biomass carbon
to total organic carbon in arable soils. Soil BioI.
Biochem. 21:471-479; 1989.
Baldock, l.A.; Oades, 1.M.; Vasallo, A.M.; Wilson, M.A.
Solid-state CPIMAS 13C NMR analysis of particle size
use a two-pool constrained model once the size of
the resistant pool is known (Paul et al. 2000).
Statistical analysis of the curves is most easily
accomplished when the CO2 evolution data are expressed on a rate per unit time basis rather than as
cumulative curves (Hess and Schmidt 1995). Use
of all the data in a single regression equation with
separate parameter estimates for each treatment
provides much greater statistical power than analyzing each treatment in a separate regression (Willson et al. in press). The effect of possible autocorrelation from repeated measurements of the same
sample also can be statistically corrected (Paul et
al. in press).
Use of Tracers
The isotopes of C include the radioactive 14C occurring in minute quantities in the atmosphere and
13C, which has a stable natural abundance of 1.1 %.
The 14C02 produced in the atmosphere by the interaction of cosmic radiation with N2 is incorporated into plants during photosynthesis at a fairly
constant rate, and can be measured with carbon dating techniques (Goh 1991). This is expensive and
not readily accessible, but together with acid hydrolysis gives the best estimates of the size and
turnover of the old resistant SOM fraction (Paul et
al. 1998; Leavitt et al. 1997). Added 14C, in the
form of uniformly labeled plant residues exposed
to a high 14C atmosphere, give exceptionally useful
information on organic matter breakdown and the
incorporation of C into microbial biomass (Wolf et
al. 1994). It is difficult to produce uniformly labeled
substrates, but field experiments can be undertaken
with commercially available, specifically labeled
substrates if adequate health and safety precautions
for working with a radioactive isotope are followed
(SchimeI1993).
The stable isotope 13C has been utilized in experiments in which plants have been 13C-Iabeled in
the laboratory in a fashion similar to that used for
radioactive elements. Reliable automated mass
spectrometers to measure the stable isotopes are
available in many laboratories. The naturally occurring discrimination against 13C02 by plants with
a C 3 photosynthetic pathway relative to those with
a C4 pathway provides a powerful, naturally occurring label in many parts of the world. This requires
a C r C 4 plant growth switch, such as the growth of
113
C 4 com or sorghum on sites previously forested
with C 3 trees, or the growth of C 3 wheat, rice, cold
season grasses, or most trees on prairie or savanna
soils that previously had C4 native vegetation. Sites
that have a mixed signal make it possible to grow
either a C 3 or C 4 test species. The 13C either enriched or depleted in the test species can be used to
measure the fate of test species plant residuesboth their incorporation into various soil biological,
chemical, and physical fractions as well as changes
in their pool sizes and flux rates.
13C studies are particularly useful when combined with CO 2 evolution studies (Paul et al. 1999).
The measurement of the 13C02 evolved during extended laboratory incubation yields estimates of the
active and slow pool sizes and decomposition kinetics for the labeled material. Details of these techniques are available in Schimel (1993), Boutton
(1996), Coleman and Fry (1991), and Paul et al.
(1998, 1999). The element N is closely associated
with soil C, and the stable isotope 15N has been
utilized in many residue decomposition and SOM
turnover studies. It, too, can provide much important information on decomposition as well as on the
microbial growth and N immobilization that occurs
during decomposition (Hauck et al. 1994).
Acknowledgments We thank a number of individuals for providing helpful discussion and insights
during the preparation of this chapter. We thank in
particular discussants at the 1996 LTER Soil Methods Standardization Workshop held in Sevilleta,
New Mexico, and especially M.E. Harmon, 1.M.
Blair, P. Sollins, E.T. Elliott, and D. Harris. This
work was supported by the National Science Foundation LTER program and the Michigan Agricultural Experiment Station.
References
Aber, J.D.; Melillo, J.M.; McClaugherty, C.A. Predicting
long-term patterns of mass loss, nitrogen dynamics,
and soil organic matter formation from initial fine litter
chemistry in temperate forest ecosystems. Can. 1. Botany 68:2201-2208; 1990.
Anderson, T.; Domsch, K.H. Ratios of biomass carbon
to total organic carbon in arable soils. Soil BioI.
Biochem. 21:471-479; 1989.
Baldock, l.A.; Oades, 1.M.; Vasallo, A.M.; Wilson, M.A.
Solid-state CPIMAS 13C NMR analysis of particle size