8. Stable Isotope Tracers and Mathematical Models in Soil Organic Matter Studies
123
of C accumulated prior to vegetation change can be
determined if the precultivation mass of C is
known:
(8.13)
where C t is the amount of C at time t, Co is the
original C content, and t is the elapsed time since
vegetation change. This application is a good
means of understanding SOM turnover times in
particular agricultural management schemes, and
has been one of the main applications of stable C
isotopes as a "tracer" in SOM cycling (Balesdent
and Mariotti 1996).
The use of SOM ol3c values following C 3 or C 4
vegetation shifts is a potentially useful, and relatively inexpensive, means of quantifying organic C
dynamics. We do not cover it in great detail here
because it has been reviewed in several recent articles (Balesdent and Mariotti 1996; Boutton,
1996). It is important here to identify several factors that may add complexity to the use of Equation
8.12: (1) the Ol3C value of atmospheric CO 2 has
decreased from - 6.5 to - 8.0%0 in the past century, changing the value of o13C p continuously over
time, (2) isotopic discrimination, or preferential
preservation of certain compounds, during decomposition may shift the true C isotope composition
of the inputs from that of o 13 C p , and (3) there is
usually a fairly wide range in the o13C values of
plants (particularly C 3 plants [Farquhar et al. 1989])
due to a variety of environmental stresses, all of
which will add uncertainty to the value assigned to
o 13 C p • While all these issues add uncertainty to the
use of this approach to quantifying C dynamics,
they do not undermine the potential of the approach
in many applications.
A related use of C isotopes is to identify vegetation shifts along forest/grassland ecotones (Wang
et al. 1993; Ambrose and Sikes, 1991) or C 3 to C4
desert shrub ecotones (e.g., Dzurec et al. 1985;
Boutton 1996; Connin et al. 1997). In these cases,
C isotopes serve in a qualitative capacity, identifying whether a vegetation shift has or has not occurred. If the timing of the shift is known, then
Equations 8.12 and 8.13 can be used to understand
the SOM turnover rates.
A recent application, more appropriate to nonagricultural settings, is the use of the known decrease in the o13C p value of atmospheric CO2 during the past 150 years (as a result ofland conversion
and fossil fuel burning) as a means of estimating
SOM turnover rates at regional or global scales.
The principle is that in the past 150 years there has
been approximately a 1.3%0 decrease in the ol3C
value of the atmosphere due to fossil fuel burning.
If this decrease is fully or partially registered in the
SOM, it implies turnover times of a century or less
(Bird et al. 1996). While this approach has been
used, in conjunction with 14C in SOM, to estimate
residence times of surficial SOM as a function of
latitude (Bird et al. 1996), some uncertainties in
turnover estimates exist due to: (1) uncertainties in
the o13C value of preindustrial SOM, (2) uncertainties in the value of the isotopic composition of the
atmosphere (and therefore plant inputs) at various
times over the past century, and (3) the extremely
small isotopic differences that are being examined
(1.3%0 or less) versus the large natural variability
in the isotopic composition of SOM. Nonetheless,
this is an innovative approach that may receive
more attention in large-scale ecological studies.
In summary, several useful methods using C isotopes to monitor SOM dynamics have been developed. Each relies on a shift in the 013C value of
plant inputs in order to derive information about
the turnover of SOM. Next we discuss more complicated models that address the intrasoil isotopic
heterogeneity.
Models for Vertical Variations in the ol3e
Value of SOM
The simple models we discuss above do not attempt
to explain the commonly observed increase in the
013C value of SOM with depth shown in Figure
8.2a (Nadelhoffer and Fry 1988; Balesdent et al.
1993). While it is possible that the fractionation
factor for SOM decomposition changes as a function of depth, the trend in 013C values versus depth
may also relate to two other phenomena: (1) depth
dependence of plant inputs combined with vertical
transport of C within the soil profile and (2) multiple pools of SOM with different C turnover rates.
For simplicity, we will discuss these two phenomena separately, in this and the next section, respectively. A model attempting a full mechanistic representation of SOC dynamics should include both
approaches (Elzein and Balesdent 1995).
It is worthwhile to note that the trend in the ol3C
values of SOM with depth is rarely a simple, mono-
123
of C accumulated prior to vegetation change can be
determined if the precultivation mass of C is
known:
(8.13)
where C t is the amount of C at time t, Co is the
original C content, and t is the elapsed time since
vegetation change. This application is a good
means of understanding SOM turnover times in
particular agricultural management schemes, and
has been one of the main applications of stable C
isotopes as a "tracer" in SOM cycling (Balesdent
and Mariotti 1996).
The use of SOM ol3c values following C 3 or C 4
vegetation shifts is a potentially useful, and relatively inexpensive, means of quantifying organic C
dynamics. We do not cover it in great detail here
because it has been reviewed in several recent articles (Balesdent and Mariotti 1996; Boutton,
1996). It is important here to identify several factors that may add complexity to the use of Equation
8.12: (1) the Ol3C value of atmospheric CO 2 has
decreased from - 6.5 to - 8.0%0 in the past century, changing the value of o13C p continuously over
time, (2) isotopic discrimination, or preferential
preservation of certain compounds, during decomposition may shift the true C isotope composition
of the inputs from that of o 13 C p , and (3) there is
usually a fairly wide range in the o13C values of
plants (particularly C 3 plants [Farquhar et al. 1989])
due to a variety of environmental stresses, all of
which will add uncertainty to the value assigned to
o 13 C p • While all these issues add uncertainty to the
use of this approach to quantifying C dynamics,
they do not undermine the potential of the approach
in many applications.
A related use of C isotopes is to identify vegetation shifts along forest/grassland ecotones (Wang
et al. 1993; Ambrose and Sikes, 1991) or C 3 to C4
desert shrub ecotones (e.g., Dzurec et al. 1985;
Boutton 1996; Connin et al. 1997). In these cases,
C isotopes serve in a qualitative capacity, identifying whether a vegetation shift has or has not occurred. If the timing of the shift is known, then
Equations 8.12 and 8.13 can be used to understand
the SOM turnover rates.
A recent application, more appropriate to nonagricultural settings, is the use of the known decrease in the o13C p value of atmospheric CO2 during the past 150 years (as a result ofland conversion
and fossil fuel burning) as a means of estimating
SOM turnover rates at regional or global scales.
The principle is that in the past 150 years there has
been approximately a 1.3%0 decrease in the ol3C
value of the atmosphere due to fossil fuel burning.
If this decrease is fully or partially registered in the
SOM, it implies turnover times of a century or less
(Bird et al. 1996). While this approach has been
used, in conjunction with 14C in SOM, to estimate
residence times of surficial SOM as a function of
latitude (Bird et al. 1996), some uncertainties in
turnover estimates exist due to: (1) uncertainties in
the o13C value of preindustrial SOM, (2) uncertainties in the value of the isotopic composition of the
atmosphere (and therefore plant inputs) at various
times over the past century, and (3) the extremely
small isotopic differences that are being examined
(1.3%0 or less) versus the large natural variability
in the isotopic composition of SOM. Nonetheless,
this is an innovative approach that may receive
more attention in large-scale ecological studies.
In summary, several useful methods using C isotopes to monitor SOM dynamics have been developed. Each relies on a shift in the 013C value of
plant inputs in order to derive information about
the turnover of SOM. Next we discuss more complicated models that address the intrasoil isotopic
heterogeneity.
Models for Vertical Variations in the ol3e
Value of SOM
The simple models we discuss above do not attempt
to explain the commonly observed increase in the
013C value of SOM with depth shown in Figure
8.2a (Nadelhoffer and Fry 1988; Balesdent et al.
1993). While it is possible that the fractionation
factor for SOM decomposition changes as a function of depth, the trend in 013C values versus depth
may also relate to two other phenomena: (1) depth
dependence of plant inputs combined with vertical
transport of C within the soil profile and (2) multiple pools of SOM with different C turnover rates.
For simplicity, we will discuss these two phenomena separately, in this and the next section, respectively. A model attempting a full mechanistic representation of SOC dynamics should include both
approaches (Elzein and Balesdent 1995).
It is worthwhile to note that the trend in the ol3C
values of SOM with depth is rarely a simple, mono-