Abstract
Abstract. Storm events are responsible for more than 60 % of the export of dissolved organic matter (DOM) from headwater catchments due to an increase in both the discharge and concentration. The latter was attributed to changing water pathways inducing the mobilization of DOM from the surface soil horizons. Recent molecular investigations have challenged this view and hypothesized (i) a contribution of an in-stream partition of organic matter (OM) between eroded particles and the dissolved fraction and (ii) the modification of the composition of soil DOM during storm events. To investigate these assumptions, soil solutions in the macropores, surface runoff and stream outlet were sampled at high frequency during three storm events in the Kervidy–Naizin catchment, part of the French critical zone observatory AgrHyS. The molecular composition of the DOM was analysed by thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH) coupled to a gas chromatograph and a quadrupole mass spectrometer. These analyses highlighted a modification of the DOM composition in soil solution controlled by the water-table dynamic and pre-event hydrological conditions. These findings fit with the mechanism of colloidal and particulate destabilization in the soil macroporosity. The different behaviour observed for lignins, carbohydrates and fatty acids highlights a potential chemical segregation based on their hydrophobicity. The composition of surface runoff DOM is similar to the DOM composition in soil solution and could be generated by the same mechanism. The DOM composition in both soil solution and surface runoff corresponds to the stream DOM composition observed during storm events. On the basis of these results, modifications of the stream DOM composition during storm events seem to be due to surface and sub-surface soil erosion rather than in-stream production.
Highlights
The transfer of organic carbon from soils to rivers and to oceans represents an important part of the global carbon cycle
7 mm of rainfall fell on 13 January, which resulted in an increase in the discharge, from 70 to 95 L s−1, and H, from 1.22 to 1.47 m
The molecular composition of dissolved organic matter (DOM) was simultaneously investigated in soil solutions, surface runoff and stream water during storm events with high-frequency sampling
Summary
The transfer of organic carbon from soils to rivers and to oceans represents an important part of the global carbon cycle. When considering annual river DOM fluxes, more than 50 % is exported during storm events (Buffam et al, 2001; Morel et al, 2009; Raymond and Saiers, 2010). This largest export is attributed to the rise of the water table in organic-rich soil horizons which become hydrologically connected to the river (Boyer et al, 1996), and to Published by Copernicus Publications on behalf of the European Geosciences Union
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
