how about:
C.B. Taylor, V.J. Fox. J. Hydrology 186 (1996) 161-190


> Dear isotopers,
> My question is the following : can we consider that the carbon isotope
> composition of soil water dissolved inorganic carbon (DIC) is mainly
> controlled by the average d13C value of the vegetation ? If not, what are
> the other sources controlling the d13C of dissolved inorganic carbon in
> soil water. I guess that soil pH, chemistry, as well as microbial activity
> must have their importance.
> Does anyone on the list knows about published or unpublished data with
d13C
> values in soil water or surface waters (rivers, lakes) DIC and co-existing
> vegetation, especially in regions where C3 and C4 biomass are available. I
> am working in the lake Chad basin and was not able to find any reference
> reporting on this.
>
> Many thanks in advance for your help!
>
> Antoine