13C composition of dissolved organic carbon in upland forested catchments of the Morvan Mountains (France): Influence of coniferous and deciduous vegetation

SummaryOne of the main environmental changes caused by human activities is that of land use. These changes influence the quantity and quality of the dissolved organic matter (DOM) fluxes through the vegetation–soil–stream system. The aim of this work is to evaluate the influence of the substitution of native deciduous forests by well managed coniferous forests on dissolved organic carbon (DOC) fluxes and their associated carbon isotopic composition (δ13CDOC). DOC fluxes and δ13CDOC were monitored for 2 years in the streams of four similar upland forested catchments in the Morvan Mountains (France). Mean annual DOC concentrations and fluxes were 2–4 times lower in streams in catchments with predominant coniferous vegetation than in those with deciduous vegetation. δ13CDOC values were lower by 0.5–1.0‰ in the stream under deciduous vegetation (−28.3‰ to −30‰) than in the other streams (−27.1‰ to −29.5‰). Furthermore, stream DOC was 13C depleted compared to the isotopic composition of the soil organic matter (SOM). This 13C depletion was higher under deciduous cover where SOM was enriched in 13C with increasing depth, producing 13C depleted solute fraction. Under coniferous cover, no significant isotopic differentiation was observed between the solute and solid phase in the soil. Our results show that the δ13CDOC in stream is marked by the vegetation cover because of differences in the degradation pathway and dynamics of SOM. Besides, it means that the substitution of deciduous forests by coniferous plantations may significantly modify the chemical composition of DOC in streams.