Tuesday, 11 July 2006 - 2:25 PM

13C Differentiation between Dissolved and Solid Organic Carbon in Soils as Induced by Substitution of a Native Deciduous Forest by a Coniferous Forest.

Philippe Amiotte Suchet, Jean Leveque, Catherine Henault, Anthony Gauthier, and Francis Andreux. Microbiologie et Géochimie des Sols - INRA Université de Bourgogne, 6 bd Gabriel, DIJON, 21000, France

In the temperate forest ecosystems, one of the main human perturbations during the last century is the substitution of native deciduous forests by well managed coniferous forests. The question of the influence of this land use change on the water quality has been addressed for a long time especially in the framework of acid rain. However, studies concerning the impact of forest management on the processes producing dissolved organic matter (DOM) are not so many. Since DOM plays an important role in aquatic ecosystems (UV filtration, pollutant transfers, nutrients and energy supply), the influence of forest cover on the quality and quantity of DOM released trough soil processes is a key question. Indeed, considering that forest cover affects the dynamic of degradation of the soil organic matter, DOM, which results from these processes, should also be affected. This work focuses on the use of the stable isotope composition of Dissolved Organic Carbon (DOC) in natural abundance to examine the changes in DOM quality produced in acid brown soils (distric cambisol) of the mountainous area of Morvan (France) under deciduous and coniferous forests. Our results show that the quality of the DOC seems to be affected by vegetation changes, as shown by the isotopic composition of dissolved and solid organic matter in the various compartments of the system. First of all, d13C of fresh aerial material do not significantly differ from one vegetation cover to another and its variability (between – 28 and -31 ‰) seems to be greater inside one plant of the same species than between plants of different species. In the same way, the d13C of organic matter from litter is not influenced by the vegetation cover (from –26 to –28 ‰) . However, the evolution of the soil organic matter d13C with increasing depth varies from one vegetation to another: soil organic matter (SOM) is enriched in 13C by about 1‰ between -5 and -55 cm under deciduous vegetation, whereas, no significant variations of d13C values are observed in SOM of profile under resinous vegetation. The isotopic composition (d13CDOC) of the dissolved organic matter (DOM) is very consistent with these patterns. Indeed, under deciduous cover, d13CDOC in litter and upper soil layer solutions are depleted in 13C by 1 to 2‰ with regard to the solid material. Thus, an apparent fractionation appears between the organic solute fraction and the solid one. Under resinous cover, no such fractionations are observed. So, the degradation of organic matter in litter and soil layers produces more 13C depleted DOC under deciduous vegetation than under resinous one. This difference can be interpreted as changes in the chemical composition of the DOM in relation with changes in the dynamic of SOM degradation and its associated microbial activity. Thus, the isotopic signature of DOC is rather controlled by the way the organic matter is degraded in the system than by the differences in the isotopic composition of organic matter. Preliminary results from soil incubation experiments seem to show that the 13C depletion observed in soil solution under deciduous vegetation could be positively related to the rate of SOM degradation.

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