TRANSPORT AND DEPOSITION OF CARBON AT CATCHMENT SCALE: STABILIZATION MECHANISMS APPROACH María Martinez-Mena*, María Almagro, Noelia Garcia-Franco1, Elvira Díaz Pereira, Carolina Boix Fayos 1. Introduction and study area 3. Results Terrestrial sedimentation buries large amounts of organic carbon (OC) annually, contributing to the terrestrial carbon sink. The temporal significance of this sink will strongly depend on the attributes of the depositional environment, but also on the characteristics of the OC reaching these sites and its stability upon deposition. The fate of the redistributed OC will ultimately depend on the mechanisms of its physical and chemical protection against decomposition, its turnover rates and the conditions under which the OC is stored in sedimentary settings. This framework is even more complex in Mediterranean river basins where sediments are often redistributed under a range of environmental conditions in ephemeral, intermittent and perennial fluvial courses, sometimes within the same catchment. The OC stabilization mechanisms and their relations with aggregation at different transport and sedimentary deposits is under those conditions highly uncertain. The main objective of this work was to characterize the stabilization and mineralization of OC in sediments in transit (suspended load and alluvial bars), deposition (reservoir sediments), and eroding soils from the source areas (soil and bank) in a sub-catchment (111 km2) at the headwaters of the Segura catchment in South East Spain (Figure 1). OC concentration in the aggregates was lower in the channel and transport areas compared to that in the soil sources, displaying reductions of about 70% in the small macro-aggregates and free micro-aggregates, and of about 45% in the free mineral fraction.   At depositional sites, reservoir sediments at 0.20 m depth showed similar aggregate distribution and OC concentration than soil sources, while reservoir sediments at deeper positions (0.20-3 m) showed similar aggregate distribution and OC concentration than the channel and transport positions. Furthermore, occluded OC (micro and mineral) represented a similar percentage of the total OC in the reservoir sediments at 0.20 m depth than in soil sources (17 and 15%, respectively; Figure 4a). C:N ratios decreased from large to small aggregates in all positions (from eroding areas to deposits) without displaying significant differences among them (high variability), except for the channel area where the highest C:N ratios were observed (Figures 2d, 2e). The concentration of OC resistant to oxidation was similar in the free and occluded mineral fractions in the soil sources, while in the channel and transport areas a much higher OC resistant to oxidation was observed in the mineral occluded fraction than in the free one (Figure 3a). From the functional point of view, transport positions showed the highest proportion of the active pool (Figure 4b) which was consistent with the highest potential mineralization rates observed (Table 1). The channel displayed the highest proportion of the slow pool, even higher than that of the soil source in which a similar proportion between active and slow pool was observed (Figure 4b). Figure 2: Distribution (a,b), OC (c,d) and C:N ratios (e, f) associated to each aggregate size class at the different catchment positions. Note that b), d), and f) show the occluded fractions within macroaggregates. Figure 1: Location of Turrilla catchment and sampling areas. (Further information can be found in Boix-Fayos et al., 2015). Figure 3: OC concentration (a) and C:N ratio (b) of the free and the occluded mineral fractions resistant to oxidation. Table 1: Soil texture, OC concentration, C:N ratio, and basal respiration in each sampling area. 2. Organic carbon fractionation approach Figure 4: Contribution of each isolated fraction (a) and functional pools (b) to the total OC at different catchment positions. 4. Summary and conclusion These preliminary results indicate that incorporation of OC in macroaggregates (250-2000µm) and free microaggregates takes place in the original soils. This incorporation promotes the occlusion of microaggregates within the macroaggregates facilitating physico-chemical mechanisms of OC protection. Those mechanisms appear also in the channel and transport sites where, besides a lower OC concentration in macroaggregates compared to the soils is observed, still microaggregates within macroaggregates are being formed and/or preserved. A good physico-chemical protection of OC in original soils determines a better physical protection of OC in sediments mobilized by fluvial transport and deposited downstream. The OC distribution and concentration in aggregates at the surface layer (0-0.20 m) of reservoir sediments was very similar to the original catchment soils. When sediments are not flooded the establishment of fresh vegetation promotes the formation of microaggregates within macroaggregates. New processes of soil formation in exposed layers of surface reservoir sediments strengthen the potential large role of those as carbon sinks. This study also emphasizes the relevance of the fractionation method proposed in order to better understand SOC dynamics as affected by erosional processes. 5. References Boix-Fayos, C., Nadeu, E., Quiñonero, J.M., Martínez-Mena, M., Almagro, M., de Vente, J. (2015). Sediment flow paths and associated organic carbon dynamics across a Mediterranean catchment, Hydrology and Earth System Sciences 19, 1209-1223. Denef, K.J., Six, J., Merckx, R., Paustian, K., 2004. Carbon sequestration in microaggregates of no-tillage soils with different clay mineralogy. Soil Science Society of America Journal 68, 1935-1944. Elliott, E.T., 1986. Aggregate structure and carbon, nitrogen, and phosphorus in native and cultivated soils. Soil Science Society of America Journal 50, 627-633. Six, J., Elliott, E.T., Paustian, K., 2000. Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture. Soil Biology & Biochemistry 32, 2099-2103 Zimmermann, M., Leifeld, J., Schmidt, N.W.I., Smith, P., Fuhrer, J., 2007. Measured soil organic matter fractions can be related to pools in the RothC model. European Journal of Soil Science 58, 658e667 Three aggregate size classes were distinguished by sieving (macroaggregates, free microaggregates, and free mineral fraction), and the microaggregates occluded within macroaggregates were isolated. As a further step, an oxidation of the OC occluded in the silt plus clay fraction, and of that in the free mineral fraction, was performed to estimate the oxidation-resistant OC pool. Measured OC in these fractions can be related to three functional pools: active (free particulate organic carbon), slow (carbon associated to clay and silt or stabilized in aggregates), and passive (oxidation-resistant OC). In addition, the potentially mineralizable C (incubation method) was determined. Soil Erosion and Conservation Group Centro de Edafología y Biología Aplicada del Segura (CEBAS-CSIC) Spanish Research Council, Murcia, Spain 1) Technical University of Munich, Freising, Germany. https://twitter.com/CEBAS_SWC www.soilwaterconservation.es www.cebas.csic.es *Corresponding author at: CEBAS-CSIC, Murcia, Spain +34.968.396371 mmena@cebas.csic.es Acknowledgement This research was financially supported by the projects SOGLO (P7/24 IAP BELSPO) from the Belgian Government and DISECO (CGL-2014-55405-R) of the Spanish CICYT. We thanks to Eloisa García and Efrain Carrillo by supporting us in the lab and field work.