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

Abstract

Anthropic changes in the drainage area of catchments can influence dominant erosion processes and sediment sources and mobilize specific carbon pools. It also causes changes in the sedimentary dynamics and thus in the fluvial morphology. At the same time fluvial morphologies can create the conditions for stabilizing organic carbon (OC) in sediments by burial, carbon preservation, slowing down mineralization processes, and terrestrial or aquatic plant colonization. All this might have a significant impact on the fluvial carbon sink or sources. This work explores the impact of changes in the drainage area (reforestation, check-dam building, agricultural abandonment) on fluvial morphology and on the sedimentary carbon sink of an arid and erodible catchment. The methodological approach combines cartographic analysis of land use, geomorphological photointerpretation of the channel and slope-channel connections in 1956 and 2016. Furthermore, soil and sediment sampling across the catchment for organic carbon stock determination was carried out. The watershed underwent a drastic transformation of land use from 1956, changing from an agrarian scenario to a forest pattern in 2016. This evolution altered sedimentological dynamics and fluvial morphologies. The active channel was narrowed (52%) whereas bank erosion (77%) and the adjacent gullies (11%) increased. The inner alluvial plain increased up to 31% and alluvial fans up to 37%. In addition, vegetation in the channel increased up to 16%. All this led to an increase of the total OC pool of fluvial sediments (12%), slightly above than the increase of OC total pool in the soils of the catchment (10%). The ratio of the OC stock sediments/soils was > 0.8, which indicates the large capacity of carbon sequestration of fluvial sediments, with OC stocks larger than those of agricultural soils. It was found that the geomorphological dynamics plays an important role in the OC fluvial flows. In scenarios of channel narrowing and vegetation encroachment of fluvial morphologies, the sediments can stabilize and generate OC sinks. These processes of OC sequestration in dry and ephemeral channels can have a large relevance for various ecosystem services and should be considered in the management of fluvial sedimentary areas.