Geoscience Reference
In-Depth Information
Plants with C3 photosynthesis have a d
13
C of -35 to -20 whereas
those with C4 photosynthesis have -17 to -9. Thus we can distinguish
the residues from these two types of plants on the basis of their isotope
contents and also calculate the proportion of plants of the two types
that have given an organic mixture with known
13
C content. Figure 1.10
gives an example of the application of this method.
100% organic carbon
Labile
organic
matter
formed
under forest
Labile
organic
matter
linked
to cropping
Stable
organic
matter
formed
under forest
0
0
Time in years since cropping started
80 years
Fig. 1.10
Variation over time of the different types of organic matter, identifi ed by their
proportion of
13
C in a soil cleared at time 0 then cultivated to sugarcane (from Scholes
et al.
1997; van Noordwijk
et al.
1997; diagrammatic).
At the end of 80 years, the stable carbon deposited under forest had
not sensibly reduced. On the other hand, the carbon with rapid turnover,
also formed under forest, disappeared in about forty years and was
replaced by carbon of the same type but characteristic of C4 plants, here
sugarcane.
The above example has allowed treatment of a problem of time in
a manner similar to what was done in the case of the isotopic dating
method. But the information on the dates here are not given by the
transformation of isotopes. It is linked to the knowledge of the history
of the land parcels. The two approaches must not be confused. But these
are composite investigations: the simultaneous use of
12
C,
13
C and
14
C
enables calculation of times and identification of the type of organic
matter.
A special issue of the journal
Geoderma
(vol. 82, nos. 1-3, 1998) is devoted
to these isotopic methods and their use in soil science. The principal
examples are given below (Table 1.2).
Other isotopic ratios used