134
biological processes) occurs in or dominates many
biologically active soils. Exceptions to this may be
coarse-textured Spodosols, where SOM is believed
to be transported downward through advection.
Previous estimates of f: have been based on rearranging Equation 33 so the data (eS z vs. In f) fit on
a line with slope € and intercept eS p • Therefore, we
suggest that previous estimates of € (Nadelhoffer
and Fry 1988; Evans and Ehleringer 1993; Piccolo
et al. 1996) may underestimate isotopic discrimination because they were based on Equation 8.33
alone. Finally, it is likely that future advection and
diffusion models that incorporate direct root inputs
will add additional complexity, but also reality, to
the analysis of depth variations in the SOM eS 15 N
values.
Ultimately, we believe that the isotopic trend
with depth for N isotopes in SOM must be described using a variation of a productiondecomposition-dispersion model (i.e., Equation
8.14) with multiple N sources and pathways, an
approach first made in early papers (Focht 1973;
Shearer et al. 1974). In the meantime, much remains to be learned of natural patterns of the rates
and isotopic composition of N inputs, the natural
patterns in SOM eS 15N values, and the rates of various soil N transformations-information needed to
constrain any model that may be constructed to explain these interesting N transformation processes.
Conclusions
With some exceptions, stable isotopes in organic
matter have been used as a tracer in SOM processes
primarily in "mixing model" approaches. In the
case of C isotopes, changes in eS l3C values, usually
resulting from C 3 /C4 shifts, allow for the quantification of rates of SOM cycling. In the case of N,
the isotopic differences between biologically fixed
N and that derived from SOM mineralization provide a means of detecting N fixation in unknown
plants or as a means of quantifying total rates of N
fixation in agricultural settings.
In contrast, the differences in the isotopic composition of SOM relative to inputs, or the common
increases in heavy isotopes with soil depth, have
not been examined with nearly the same intensity.
We believe that these observations can be analyzed
with models of varying complexity with rewarding
Ronald Amundson and W. Troy Baisden
results. First, in the case of N, if soils are considered
to be well-mixed pools of SOM, comparisons of
inputs to the SOM pool at steady state can lead to
important insights into the similarities or differences in the processes of N additions and losses in
contrasting environments. For both C and N, the
widely ranging increases in eS values with increasing depth hint at interesting combinations of processes, which may be ultimately captured by analytical or numerical mathematical models.
As the interest in ecosystem functioning grows,
isotopes combined with mathematical modeling
will play an important role in understanding the
type and magnitude of SOM cycling processes. Despite nearly 30 years of research on SOM isotope
chemistry, this is still a largely unexplored field of
research-one which will provide many challenging opportunities for years to come.
Acknowledgments This research was supported by
a NASA Earth System Science Fellowship to W.T.
Baisden and by University of California Agricultural Experiment Station, University of California
Committee on Research, and Kearney Foundation
of Soil Science funding to R. Amundson. We thank
Professor Sue Trumbore for her review of the chapter and Professor Peter Vitousek, Kitty Lohse, and
Dana Brenner for helpful comments on an earlier
version of the manuscript.
References
Ambrose, S.H.; Sikes, N.E. Soil carbon isotope evidence
for Holocene habitat change in the Kenya Rift Valley.
Science 252:1402-1405; 1991.
Amundson, R; Chadwick, O.A.; Sowers, J.M.; Doner,
H.E. Soil evolution along an altitudinal transect in the
eastern Mojave Desert of Nevada, U.S.A. Geoderma
43:349-371; 1989.
Amundson, R.; Stem, L.; Baisden, T.; Wang, Y. The isotopic composition of soil and soil-respired CO2, Geoderma 82:83-114, 1998.
Arrouays, D.; Balesdent, J.; Mariotti, A.; Girardin, C.
Modeling organic carbon turnover in cleared temperate forest soils coverted to maize cropping by using
13C natural abundance measurements. Plant Soil
173:191-196; 1995.
Austin, A.T.; Vitousek. P.M. Nutrient dynamics on a precipitation gradient in Hawaii. Oecologia 113:519529; 1998.
biological processes) occurs in or dominates many
biologically active soils. Exceptions to this may be
coarse-textured Spodosols, where SOM is believed
to be transported downward through advection.
Previous estimates of f: have been based on rearranging Equation 33 so the data (eS z vs. In f) fit on
a line with slope € and intercept eS p • Therefore, we
suggest that previous estimates of € (Nadelhoffer
and Fry 1988; Evans and Ehleringer 1993; Piccolo
et al. 1996) may underestimate isotopic discrimination because they were based on Equation 8.33
alone. Finally, it is likely that future advection and
diffusion models that incorporate direct root inputs
will add additional complexity, but also reality, to
the analysis of depth variations in the SOM eS 15 N
values.
Ultimately, we believe that the isotopic trend
with depth for N isotopes in SOM must be described using a variation of a productiondecomposition-dispersion model (i.e., Equation
8.14) with multiple N sources and pathways, an
approach first made in early papers (Focht 1973;
Shearer et al. 1974). In the meantime, much remains to be learned of natural patterns of the rates
and isotopic composition of N inputs, the natural
patterns in SOM eS 15N values, and the rates of various soil N transformations-information needed to
constrain any model that may be constructed to explain these interesting N transformation processes.
Conclusions
With some exceptions, stable isotopes in organic
matter have been used as a tracer in SOM processes
primarily in "mixing model" approaches. In the
case of C isotopes, changes in eS l3C values, usually
resulting from C 3 /C4 shifts, allow for the quantification of rates of SOM cycling. In the case of N,
the isotopic differences between biologically fixed
N and that derived from SOM mineralization provide a means of detecting N fixation in unknown
plants or as a means of quantifying total rates of N
fixation in agricultural settings.
In contrast, the differences in the isotopic composition of SOM relative to inputs, or the common
increases in heavy isotopes with soil depth, have
not been examined with nearly the same intensity.
We believe that these observations can be analyzed
with models of varying complexity with rewarding
Ronald Amundson and W. Troy Baisden
results. First, in the case of N, if soils are considered
to be well-mixed pools of SOM, comparisons of
inputs to the SOM pool at steady state can lead to
important insights into the similarities or differences in the processes of N additions and losses in
contrasting environments. For both C and N, the
widely ranging increases in eS values with increasing depth hint at interesting combinations of processes, which may be ultimately captured by analytical or numerical mathematical models.
As the interest in ecosystem functioning grows,
isotopes combined with mathematical modeling
will play an important role in understanding the
type and magnitude of SOM cycling processes. Despite nearly 30 years of research on SOM isotope
chemistry, this is still a largely unexplored field of
research-one which will provide many challenging opportunities for years to come.
Acknowledgments This research was supported by
a NASA Earth System Science Fellowship to W.T.
Baisden and by University of California Agricultural Experiment Station, University of California
Committee on Research, and Kearney Foundation
of Soil Science funding to R. Amundson. We thank
Professor Sue Trumbore for her review of the chapter and Professor Peter Vitousek, Kitty Lohse, and
Dana Brenner for helpful comments on an earlier
version of the manuscript.
References
Ambrose, S.H.; Sikes, N.E. Soil carbon isotope evidence
for Holocene habitat change in the Kenya Rift Valley.
Science 252:1402-1405; 1991.
Amundson, R; Chadwick, O.A.; Sowers, J.M.; Doner,
H.E. Soil evolution along an altitudinal transect in the
eastern Mojave Desert of Nevada, U.S.A. Geoderma
43:349-371; 1989.
Amundson, R.; Stem, L.; Baisden, T.; Wang, Y. The isotopic composition of soil and soil-respired CO2, Geoderma 82:83-114, 1998.
Arrouays, D.; Balesdent, J.; Mariotti, A.; Girardin, C.
Modeling organic carbon turnover in cleared temperate forest soils coverted to maize cropping by using
13C natural abundance measurements. Plant Soil
173:191-196; 1995.
Austin, A.T.; Vitousek. P.M. Nutrient dynamics on a precipitation gradient in Hawaii. Oecologia 113:519529; 1998.