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

Abstract

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 sub-humid 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 20 contribution of transport and depositional areas as sources or sinks of CO 2 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 the main channel as suspended load and bedload; and iii) depositional areas along the channel, downstream in alluvial wedges and in the reservoir at the outlet of the 25 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 physico-chemical 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 medium-term depositional areas (alluvial wedges) was mainly driven by physical stabilization mechanisms, while chemical protection of OC was observed in the fluvial lateral banks. However, in the 30 remaining sediment sources, in sediments during transport, and after deposition in long-term deposits (the reservoir), both mechanisms are equally relevant. Mineralization of the most labile OC, intra-aggregate particulate organic matter (MPOM), was predominant during transport. Aggregate formation and OC accumulation, mainly associated with macroaggregates and occluded microaggregates within macroaggregates, were predominant in depositional areas, being more protected than the OC from the most eroding sources (agricultural soils and fluvial lateral banks). Both temporary and permanent sediment 35 Biogeosciences Discuss., https://doi.

Presión antrópica sobre cuencas de drenaje en ecosistemas frágiles: variaciones en las existencias (stock) de carbono orgánico asociadas a cambios morfológicos fluviales

“…Las distintas fuentes de sedimentos (suelos forestales, agrícolas, márgenes laterales, el propio cauce, etc.) proveen distintos reservorios de CO y los procesos erosivos se encargan de seleccionarlo durante las fases de arranque, transporte y sedimentación (Boix-Fayos et al, 2015;Doetterl et al, 2016, Martínez-Mena et al, 2018 (Fig. 1).…”

“…Además, la dinámica sedimentaria fluvial, la frecuencia de los eventos y magnitud del caudal y la carga sedimentaria, condicionan las formas fluviales en el cauce y las fases de estabilidad, de agradación o de incisión. En estas fases la vegetación puede o no establecerse, determinando el enterramiento, mineralización o entrada de nuevo carbono (autóctono o alóctono) a los sedimentos (Martínez-Mena et al, 2018).…”