The transfers and origins of dissolved inorganic carbon DIC. were studied for a year in a soil–spring–stream system in
the Strengbach catchment, Vosges mountains, France. This 80 ha experimental research basin is located on the eastern side
of the mountains, at an altitude ranging from 883 to 1146 m.a.s.l. and is mainly covered by spruce 80%.. Brown acid and
podzolic soils developed on a granitic basement, and, as a result, the DIC originates solely from CO2 generated by oxidation
of soil organic matter. The d13CDIC. in catchment waters is highly variable, from about y22‰ in the springs and
piezometers to about y12‰ in the stream at the outlet of the catchment. In the springs, pronounced seasonal variations of
d13C exist, with the DIC in isotopic equilibrium with the soil CO that has estimated d13DIC 2 C of about y24‰ in winter and
y20‰ in summer. These seasonal variations reflect an isotopic fractionation that seems only induced by molecular
diffusion of soil CO2 in summer. In stream water, seasonal variations are small and the relatively heavy DIC y12‰ on
average. is a result of isotopic equilibration of the aqueous CO2 with atmospheric CO2
