Assessing the Role of Oxbow Deposits in Carbon and Nitrogen Dynamics within Alpine Watersheds

Abstract

Despite their abundance, floodplains are poorly understood components of the global carbon cycle that are not well represented in Earth system models. The hydrology of floodplains was thought to be the major factor in controlling the chemical state and microbial activity within their sediments and groundwater. This paper seeks to characterize the impact of changing water levels on carbon and nitrogen cycling within alpine floodplains. This study took place in a meander in the East River floodplain in Central Colorado, where the summer drawdown causes a rapid decline in the floodplain’s groundwater. Although carbon and nitrogen data have not been received at the time of writing, reduced iron was found to increase throughout the meander over time even as groundwater levels fell, with an unexpected peak in the center of the meander. This peak suggests that there is also a peak in metabolized organic carbon in the same location, which tends to drive the reduction of iron. We expect that ammonium (reduced nitrogen) will follow the same trend as reduced iron, while nitrate should follow the opposite trend, as nitrogen and iron are both redox-active. Dissolved oxygen was low in the center of the meander but occurred in higher concentrations along the riverbanks. Plants in the floodplain, which can oxygenate the subsurface via their roots, may be the cause of the oxicity of the downstream edge of the meander. Overall, it is clear that there are other major controls on floodplain chemical cycling beyond hydrology.

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