Carbon and nitrogen accretion in the topsoil of the Middle Ebro River Floodplains (NE Spain): Implications for their ecological restoration

This study aimed at evaluating the potential of floodplains in the Middle Ebro River (Spain) to accumulate organic carbon and nitrogen through sedimentation. Total organic carbon (TOC) and nitrogen (TN) in the top soil of riparian habitats and in the river sediments of one river reach were examined and related to landform and land use to gain insight in the current patterns of soil organic matter accumulation. Based on our results, the potential of floodplain soils as OM sinks was assessed to propose a rehabilitation framework which includes carbon and nitrogen accumulation.To achieve those goals, six categories of landform evolution, including crops and poplar groves, were defined using aerial photographs. The study plots (n = 18), one per category, were set in three areas of the study reach. Soil organic matter (SOM) quantity (TOC, TN) and quality (C:N, non-hydrolizable carbon) were characterized for each plot. With respect to the river sediments, the material deposited after one flood was analysed to estimate the relationship between grain size and organic matter (OM) content. It was used to infer the relative importance of allochthonous vs. autochthonous OM inputs in the top soil of the study plots. According to our results, landform evolution influences the quantity but not the quality of top soil OM in the Middle Ebro floodplains. Natural patches >60-year-old incorporated in situ produced organic matter and presented the highest OC and N stocks. In turn, sedimentation was the dominant process in SOM dynamics at younger natural patches. Furthermore, approximately half of the OC could be included within the passive pool. In any case, anthropogenic land use counteracts the ability of floodplain soils to act as nitrogen and carbon sinks; thus, the rehabilitation of the floodplain towards natural land covers is required.