Abstract
Abstract. The transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of micropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochemistry. DOM delivery to streams during storms is assumed to come from the flushing of preexisting soil DOM reservoirs mobilized by the modification of water flow paths. We tested this hypothesis by investigating the evolution of the composition of stream DOM during inter-storm conditions and five storm events monitored with high-frequency sampling. The composition of DOM was analyzed using thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer. In inter-storm conditions, stream DOM is derived from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms due to the increase in water velocity, resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks, leading to a partition of organic matter between particulate and dissolved phases. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partition between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of the water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. These results are the most important factors in understanding the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events.
Highlights
The transfer of dissolved organic matter (DOM) across soil– river interfaces is a globally relevant carbon flux (Cole et al, 2007) and a major control on the biogeochemistry of micropollutants (Corapcioglu and Jiang, 1993; Raymond et al, 2013)
Jeanneau et al.: Sources of dissolved organic matter during storm and inter-storm conditions scape with surface waters, more than 60 % of annual dissolved organic carbon (DOC; the parameter commonly used to quantify DOM concentration) load can occur during storm events (Morel et al, 2009)
The ratios can differentiate between stream DOM and inter-storm and from storm conditions (Dalzell et al, 2005; Hernes et al, 2008), leading to the assumption of stream DOM produced by an erosive process
Summary
The transfer of dissolved organic matter (DOM) across soil– river interfaces is a globally relevant carbon flux (Cole et al, 2007) and a major control on the biogeochemistry of micropollutants (Corapcioglu and Jiang, 1993; Raymond et al, 2013). Organic matter sources are typically abundant in headwater catchment soil, meaning that DOM flux depends primarily on water flow path (McDonnell, 2003; Morel et al, 2009), which changes on seasonal and event scales in response to hydroclimatic conditions (Hinton et al, 1998). L. Jeanneau et al.: Sources of dissolved organic matter during storm and inter-storm conditions scape with surface waters, more than 60 % of annual dissolved organic carbon (DOC; the parameter commonly used to quantify DOM concentration) load can occur during storm events (Morel et al, 2009). DOC concentration typically increases during storm events as elevated water table and enhanced near-surface flow cause the leaching of DOM-rich soil horizons (Maurice et al, 2002; McGlynn and McDonnell, 2003)
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