8. Stable Isotope Tracers and Mathematical Models in Soil Organic Matter Studies
119
ganic N [DON]), and (4) erosion. While plant uptake rates can be measured (see Chapter 11), the
remaining fluxes remain poorly quantified in most,
but not all (Riley and Vitousek, 1995), soil systems.
Internal SOM Transfers
The magnitude of the direct downward transport of
surficial litter into the underlying soil remains
largely unknown in many environments. In some
ecosystems with large earthworm populations, direct incorporation of surface litter into the mineral
soil pool represents a very important and quantifiable process (Darwin 1881). The influence of other
mesofauna on litter incorporation is possibly less
well quantified.
It is well accepted that in certain soils, such a
Spodosols, downward transport of organic molecules with water constitutes the main, or in some
cases the only, source of organic matter for certain
horizons (e.g., Bhs or Bh horizons). However, the
importance in other soils is far less constrained. Recent modeling of 14C, cesium-137 ( 137 Cs), and lead210 elOPb) in some forest soils of Germany have
placed some constraints on rates of downward
transport in certain environments (Dorr and Miinnich 1989); however, this approach needs to be expanded to other ecosystems and soil environments
to provide a general framework for modeling
purposes.
SOM Transformations
The fate of plant detritus in soils continues to be an
area of active research involving many unresolved
issues concerning the pathways of organic matter
decay (Oades 1995). A common simplification of
modeling these complex C cycling processes is to
assume that decomposition of organic compounds,
and the resulting production of CO 2 and various
humic substances, is a first order decay reaction
(e.g., Cerling et al. 1991; Trumbore 1993; Wang et
al. 1994). The CO 2 produced in this process diffuses to the atmosphere (Cerling et al. 1991 ;
Amundson et al. 1998) while the remaining humic
substances can undergo further modification or be
stabilized in relatively nonreactive states or sites.
The decay constant, k, while assumed to be constant with depth and time in relatively simple models, is likely a function of both variables.
Similar modeling approaches may be taken for
soil N, although the reactions and products are considerably more numerous than for C (Schlesinger
1991). Soil organic N is mineralized to NHt which
can either undergo plant uptake, microbial immobilization, volatilization, or nitrification. During nitrification, NHt is oxidized to NO; , although N 2 0
may also be produced during this process (Goreau
et al. 1980). Finally, oxidized forms of N can be
reduced ultimately to N2 during the process of denitrification. As we discuss later in this chapter, the
modeling of these diverse processes in SOM N isotope studies has not been attempted, but various
simplifying approaches can be made.
Stable Carbon Isotopes
in Organic Matter
The central problem in understanding the temporal
and spatial isotopic variations in soil organic matter
(regardless of the isotope being considered) is the
reality that organic compounds are not homogeneous isotopically or chemically. Plant compounds
(the ultimate source of most soil organic matter)
vary greatly structurally (e.g., lipids, cellulose, lignin, etc.), and each structural component can have
widely varying stable isotope ratios. Additionally,
SOM is likely even more heterogeneous, with
widely varying susceptibilities to decomposition
and (in particular for 14C) widely differing isotopic
ratios.
Below we examine two contrasting approaches
to modeling C isotopes in SOM, differing in the
number of processes captured and in the resulting
mathematical complexity. Each of these contrasting
approaches can provide important insights into C
cycling-allowing comparisons between ecosystems or determining depth-dependent processes
within a given soil. This discussion also serves as
the introduction for the modeling of N isotopes in
SOM.
Well-Mixed One Box Model of C
Isotopes in SOM
As a simplifying assumption for any SOM horizon,
or for the soil profile as a whole, the mass of organic
matter can be thought of as a well-mixed pool that
119
ganic N [DON]), and (4) erosion. While plant uptake rates can be measured (see Chapter 11), the
remaining fluxes remain poorly quantified in most,
but not all (Riley and Vitousek, 1995), soil systems.
Internal SOM Transfers
The magnitude of the direct downward transport of
surficial litter into the underlying soil remains
largely unknown in many environments. In some
ecosystems with large earthworm populations, direct incorporation of surface litter into the mineral
soil pool represents a very important and quantifiable process (Darwin 1881). The influence of other
mesofauna on litter incorporation is possibly less
well quantified.
It is well accepted that in certain soils, such a
Spodosols, downward transport of organic molecules with water constitutes the main, or in some
cases the only, source of organic matter for certain
horizons (e.g., Bhs or Bh horizons). However, the
importance in other soils is far less constrained. Recent modeling of 14C, cesium-137 ( 137 Cs), and lead210 elOPb) in some forest soils of Germany have
placed some constraints on rates of downward
transport in certain environments (Dorr and Miinnich 1989); however, this approach needs to be expanded to other ecosystems and soil environments
to provide a general framework for modeling
purposes.
SOM Transformations
The fate of plant detritus in soils continues to be an
area of active research involving many unresolved
issues concerning the pathways of organic matter
decay (Oades 1995). A common simplification of
modeling these complex C cycling processes is to
assume that decomposition of organic compounds,
and the resulting production of CO 2 and various
humic substances, is a first order decay reaction
(e.g., Cerling et al. 1991; Trumbore 1993; Wang et
al. 1994). The CO 2 produced in this process diffuses to the atmosphere (Cerling et al. 1991 ;
Amundson et al. 1998) while the remaining humic
substances can undergo further modification or be
stabilized in relatively nonreactive states or sites.
The decay constant, k, while assumed to be constant with depth and time in relatively simple models, is likely a function of both variables.
Similar modeling approaches may be taken for
soil N, although the reactions and products are considerably more numerous than for C (Schlesinger
1991). Soil organic N is mineralized to NHt which
can either undergo plant uptake, microbial immobilization, volatilization, or nitrification. During nitrification, NHt is oxidized to NO; , although N 2 0
may also be produced during this process (Goreau
et al. 1980). Finally, oxidized forms of N can be
reduced ultimately to N2 during the process of denitrification. As we discuss later in this chapter, the
modeling of these diverse processes in SOM N isotope studies has not been attempted, but various
simplifying approaches can be made.
Stable Carbon Isotopes
in Organic Matter
The central problem in understanding the temporal
and spatial isotopic variations in soil organic matter
(regardless of the isotope being considered) is the
reality that organic compounds are not homogeneous isotopically or chemically. Plant compounds
(the ultimate source of most soil organic matter)
vary greatly structurally (e.g., lipids, cellulose, lignin, etc.), and each structural component can have
widely varying stable isotope ratios. Additionally,
SOM is likely even more heterogeneous, with
widely varying susceptibilities to decomposition
and (in particular for 14C) widely differing isotopic
ratios.
Below we examine two contrasting approaches
to modeling C isotopes in SOM, differing in the
number of processes captured and in the resulting
mathematical complexity. Each of these contrasting
approaches can provide important insights into C
cycling-allowing comparisons between ecosystems or determining depth-dependent processes
within a given soil. This discussion also serves as
the introduction for the modeling of N isotopes in
SOM.
Well-Mixed One Box Model of C
Isotopes in SOM
As a simplifying assumption for any SOM horizon,
or for the soil profile as a whole, the mass of organic
matter can be thought of as a well-mixed pool that