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

One 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 welt managed coniferous forests on dissolved organic carbon (DOC) fluxes and their associated carbon isotopic composition (delta C-13(DOC)). DOC fluxes and delta C-13(DOC) 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 tower in streams in catchments with predominant coniferous vegetation than in those with deciduous vegetation. delta C-13(DOC) values were lower by 0.5-1.0 parts per thousand in the stream under deciduous vegetation (-28.3 parts per thousand to -30 parts per thousand) than in the other streams (-27.1 parts per thousand to -29.5 parts per thousand). Furthermore, stream DOC was C-13 depleted compared to the isotopic composition of the soil organic matter (SOM). This C-13 depletion was higher under deciduous cover where SOM was enriched in C-13 with increasing depth, producing C-13 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 delta C-13(DOC) 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. (c) 2006 Elsevier B.V. All rights reserved.