Fossil organic matter in modern soils : optical evidences.

Soil organic matter is one of the key components in the global carbon cycle understanding. Its description and quantification has then been subject to many investigations. The present work is driven by the following question: does soil organic matter (OM) only depends on the vegetal net primary production or also on the incorporation of organic matter coming from underlying geological formations? Brief replies were provided by the comparative study of particulate OM present in the rocks (marl and limestone) and the soils of the catchment area of Annecy Lake (French Alps). About twenty samples of soils and underlying rocks were studied by optical study that consists of counting of organic particles isolated from the mineral matrix after acid treatment. Observations were performed in natural light in both transmitted (palynofacies) and reflected modes (vitrinite reflectance). Observation in transmitted light shown that the geological formations OM is almost made of opaque particles (OP). In soils, OM consists of both well preserved translucent particles resulting from the overlying primary production and variable amounts of OP (1- 11% of total OM). In the rocks, the OP size distribution presents mean values around 20 micrometre. This size fraction is also found in all the studied soils. These observations clearly indicate that ~20 micrometre OP particles observed in the soils are derive from underlying sedimentary rocks. However, for 50% of the soils considered, some OP particles with mean size values around 60 micrometre are also found. This fraction is never observed in the rocks of the underlying rocks, indicating that these particles cannot derive from the geological substrate. Moreover, the observations in reflected light show that the ~ 60 micrometre particles from the soils exhibit common features of wood fire debris (pyrofusinite). Consequently, in some cases, in addition to primary production derived particles, the soils may also contain opaque OM coming from both sedimentary rocks and from forest fires. These results show : - that altered OM of the geological formations can be found in modern soils and then must be taken into account in the understanding of the carbon cycle models ; - the possibility to distinguish fossil OM (geological OM) from present OM (particles from present biomass and fire forest)