Anthropogenic legacy effects control sediment and organic carbon storage in temperate river floodplains

Abstract

Rivers convey mineral sediment and organic carbon through the landscape but also store a significant part in floodplains for varying amounts of time. Reliable quantification of this storage component is necessary to understand the cascades and land-ocean transfers of sediment and organic carbon. Here, we quantified the Holocene floodplain sediment and organic carbon storage for four medium-sized rivers in Belgium, located in the European loess and sand belts. The results indicate that floodplains store large amounts of mineral sediment and soil organic carbon with most of the storage at greater depths below the surface. The observed regional variability in floodplain storage shows to be controlled by catchment characteristics and anthropogenic impact. The river basins which have known long-term anthropogenic impact do not only store on average 4.4 times more sediment, but also 2.6 times more organic carbon. Even more, the historic human impact trajectory has a profound impact on the sensitivity of the organic carbon stock to external factors in current times. The observed variability in floodplain storage, both within and between river basins, shows to be large, even for river basins which have experienced similar environmental conditions, obscuring simple scale relations that would allow budgeting of global floodplain sediment and carbon stocks under anthropogenic impact.