1. Fate and Transport of Dissolved Organic Carbon in Soils from Two Contrasting Watersheds Oak Ridge National Laboratory, Environmental Sciences Division J.R. Tarver, J. A. Palmer, P.M. Jardine, D.E. Todd, B. Kinsall Batch sorption isotherms on soils from the pedons suggested that Ultisols tended to sorb more organic C as a function of depth relative to Inceptisols. Variability in organic C sorption was a function of solid phase pH, indigenous sorbed organic C, and clay content. Inceptisol Ultisol Inceptisol C = macropores M = mesopores F = micropores Ultisol M = mesopores F = micropores Abstract The purpose of this research is to provide an improved understanding and predictive capability of the mechanisms that control the transport and immobilization of organic C through soil profiles. The study is motivated by the likelihood that deep clay and Fe-oxide rich subsoils of humid and tropical climates have a tremendous capacity to stabilize and accumulate organic C thus decreasing C turnover rates by orders of magnitude relative to surface soils. Our approach utilizes two well-characterized, highly instrumented field pedons on contrasting watersheds of an Inceptisol and an Ultisol. The pedons are equipped with multiporosity sampling capabilities as a function of depth, and organic C and solute fluxes are quantified in multiple pore domains during storm drainage through the profiles. The inceptisol exhibited the highest C flux during storm events which is consistent with its more rapid flow and transport characteristics and lower organic C retention capacity. The more highly weathered Ultisol exhibited significantly lower organic C fluxes, which may be related to their higher organic C retention capacity. Both physical and geochemical processes appear to control the mobility of the organic C through the soil profiles. The significance of these deep soil C credits towards offsetting increased anthropogenic CO 2 emissions will ultimately be evaluated. Background Two highly instrumented in situ soil blocks of contrasting soil types are being used to test the ability of organic C accumulation in subsurface soils. Through these soil blocks we can gauge the impact of hydrological, geochemical, and microbial processes on subsoil organic C sequestration. Such data will be useful in the identification of regions and field sites which offer the greatest potential for enhanced subsurface organic C storage and thus most deserving of manipulation or innovative management. Lower horizons Anthropogenically enriched soil of the Amazon Adjacent nonenriched soil These xantic Ferralsols are from the same physiographic position and have the same clay content and clay mineralogy. The soil on the left was purposely enriched with dissolved organic C by ancient human occupation centuries ago. This provides an indication that such enrichments can be maintained at a minimum of several centuries. An example of sustained subsoil organic C sequestration Approach Ultisol Inceptisol Two well-characterized, highly instrumented field pedons on contrasting watersheds of an Inceptisol and an Ultisol. The pedons are equipped with multiporosity sampling capabilities as a function of depth. Organic C and solute fluxes were quantified in macro-, meso-, and micropore domains during storm drainage through the profiles. From Sombroek et al., 1993 Highly weathered subsoils often contain appreciable clay that is heavily coated with amorphous and crystalline Fe-oxides. Fe-oxide coatings on mineral surfaces strongly sequester pore water organic C. The recalcitrant organic C storage capacity of these soils can potentially be increased several fold. The organic C is strongly bound by the solid phase which limits bioavailability and transport to groundwater. Organic C sorption on soil – Importance of Fe-oxides High clay, high Fe-oxide content Low clay, low Fe-oxide content Ultisols High Fe-oxides Low Fe-oxides Lower horizons Upper horizons Pore water was frequently fractionated into hydrophilic and hydrophobic acid and neutral species and analyzed for its aromaticity. Organic C sorption isotherms were also perfomed on soils acquired from the pedons as a function of depth. Depth (cm) 17 90 20 22 36 55 56 84 85 120 117 146 177 208 236 Depth (cm) 17 29 53 67 104 90 131 177 149 The highly fractured Inceptisol exhibited the highest C flux during storm events which is consistent with its more rapid flow and transport characteristics and lower organic C retention capacity. Mesopore domains along dipping bedding planes served as conduits for organic C movement through the profile. The more highly weathered Ultisol exhibited significantly lower organic C fluxes which may be related to their higher organic C retention capacity. Micropore storage capacity similar to Inceptisol which may be related to similar clay content of the two soils. Lower aromaticity indicates less humic rich organic C in the pore water suggesting that these larger organic molecules are being preferentially adsorbed by the solid phase during movement through the profile. Similar pore water aromaticity values observed for both the Inceptisol and Ultisol. Both soil types show decreasing aromaticity with increasing depth suggesting preferential loss of larger organic molecules during transport. Dip Both the Incetisol and Ultisol show a decreasing pore water hydrophobic acid content indicating that larger organic C molecules are being preferentially adsorbed by the solid phase during movement through the profile. These results are consistent with the aromaticity results. The Ultisol also shows a decreasing total hydrophobic content with depth suggesting preferential sorption of both acidic and neutral organic macromolecules. Conclusions: Storm driven transport of organic C through an Ultisol and Inceptisol suggested that both physical and geochemical processes control fate and transport of C through the soil profiles. The highly fractured Inceptisol exhibited the highest C flux during storm events which is consistent with its more rapid flow and transport characteristics and lower organic C retention capacity relative to the Ultisol. Mesopore domains along dipping bedding planes served as conduits for organic C movement through the profile. Both aromaticity and hydrophobicity measurements suggested that larger organic C molecules were being preferentially adsorbed by the solid phase during movement through the profile. These results provide quantitative information on the significance of carbon credits in deep soil profiles.