Stable hydrogen isotope ratios in soil constituents: New insights into the formation of organic matter and clay minerals

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The use of stable hydrogen isotope ratios (delta2H values) in soils has up to now been limited, because part of the H pool in soil exchanges with ambient moisture and does not carry a meaningful isotope signal. This part needs to be separated from the nonexchangeable part which contains source and process information. For soil organic matter (SOM), steam equilibration of demineralized SOM with waters of known H isotope composition can be used while for clay minerals degassing at ca. 200°C under vacuum is common to separate exchangeable and nonexchangeable H. Previous work has suggested that SOM may be rapidly processed through the microorganisms during which C-bound H is exchanged in the glycolytic metabolism. Clay minerals conserve the delta2H value of ambient water at the time of formation in structural OH. We propose to study (i) the incorporation of ambient water H into the nonexchangeable H fraction (C-H bonds) in SOM and the associated isotopic fractionation during decomposition (ii) whether the soil clay fraction can be used to determine the origin of a soil sample and for relative dating of argillic B (Bt) horizons of recent soils and palaeosols. We will conduct laboratory and field incubations, establish steam equilibration for clay fractions, and analyze clay fractions of a global set of soils and of partly dated palaeo Bt horizons.