Fluvial sedimentary deposits as carbon sinks: Organic carbon pools and stabilization mechanisms across a Mediterranean catchment

Abstract

p'The role of fluvial sedimentary areas as organic carbon sinks remains largely unquantified. Little is known about mechanisms of organic carbon (OC) stabilization in alluvial sediments in semiarid and subhumid catchments where those mechanisms are quite complex because sediments are often redistributed and exposed to a range of environmental conditions in intermittent and perennial fluvial courses within the same catchment. The main goal of this study was to evaluate the contribution of transport and depositional areas as sources or sinks of 'span classCombining double low line"inline-formula"'CO2'/span' at the catchment scale. We used physical and chemical organic matter fractionation techniques and basal respiration rates in samples representative of the three phases of the erosion process within the catchment: (i) detachment, representing the main sediment sources from forests and agricultural upland soils, as well as fluvial lateral banks; (ii) transport, representing suspended load and bedload in the main channel; and (iii) depositional areas along the channel, downstream in alluvial wedges, and in the reservoir at the outlet of the catchment, representative of medium-and long-Term residence deposits, respectively. Our results show that most of the sediments transported and deposited downstream come from agricultural upland soils and fluvial lateral bank sources, where the physicochemical protection of OC is much lower than that of the forest soils, which are less sensitive to erosion. The protection of OC in forest soils and alluvial wedges (medium-Term depositional areas) was mainly driven by physical protection (OC within aggregates), while chemical protection of OC (OC adhesion to soil mineral particles) was observed in the fluvial lateral banks. However, in the remaining sediment sources, in sediments during transport, and after deposition in the reservoir (long-Term deposit), both mechanisms are equally relevant. Mineralization of the most labile OC (the intra-Aggregate particulate organic matter ('span classCombining double low line"inline-formula"''i'M'/i'pom'/span') was predominant during transport. Aggregate formation and OC accumulation, mainly associated with macroaggregates and occluded microaggregates within macroaggregates, were predominant in the upper layer of depositional areas. However, OC was highly protected and stabilized at the deeper layers, mainly in the long-Term deposits (reservoir), being even more protected than the OC from the most eroding sources (agricultural soils and fluvial lateral banks). Altogether our results show that both medium-and long-Term depositional areas can play an important role in erosive areas within catchments, compensating for OC losses from the eroded sources and functioning as C sinks. © Author(s) 2019.