Dissolved organic matter in soil: challenging the paradigm of sorptive preservation

During the last decade, research in sedimentary systems led to the paradigm of sorptive stabilization of organic matter (OM). Studies on soils also show that sorptive interactions between dissolved organic matter (DOM) and mineral phases contribute to the preservation of soil OM. In the first part of the paper, we summarize evidence for sorptive stabilization of OM in forest soils including (a) pronounced retention of DOM in most subsoils, (b) strong chemisorptive binding exhibiting strong hysteresis, and (c) similarity in the composition of DOM and OM in illuvial soil horizons and clay-sized separates. However, the capacity of soils for sorption of DOM is not infinite. In the second part of the paper, we present a case study where we relate the yearly retention of dissolved organic carbon (DOC) in the mineral soil to the available sorption capacity of seven forest soils. We estimate that the saturation of the sorption complex would occur within 4–30 years. Assuming these soils are in steady-state equilibrium with respect to carbon cycling, this suggests a mean residence time of the sorbed organic carbon (OC) of about the same time, therefore providing little evidence for a long-term stabilization of sorbed OM. One explanation for this discrepancy may be because in forest soils most surfaces are not characterized by juvenile minerals but are covered with OM and colonized by microorganisms. This is the case mainly in topsoil horizons but occurs also along preferential flow paths and on aggregate surfaces. Biofilms develop particularly at sites receiving high input of nutrients and organic substrates, i.e., DOM, such as preferential flow paths. The OM input enhances the heterotrophic activity in the biofilm, converting the DOM into either organic compounds by microbial resynthesis or inorganic mineralization products. Recent studies suggest that Fe hydrous oxides embedded within the biofilms may serve as a sorbent and shuttle for dissolved organic compounds from the surrounding aqueous media. We assume that sorption of DOM to the biofilm does not lead to a stabilization of OM but is a prerequisite for its rapid turnover. Only when DOM is transported by mass flow or diffusion to fresh, juvenile mineral surfaces, may sorption effectively stabilize OM. This stabilization would involve complexation of functional groups, changed conformation, and incalation in small pores.