Storm dissolved organic matter : surface and sub-surface erosion controls its composition

International audienceIn headwater catchments, flood events are responsible for exportation of the major part of DOM (dissolved organicmatter) during the hydrological year. During these hot moments, the increased flow at the outlet is accompaniedwith an increase of DOM concentrations, implying the mobilisation of additional DOM sources which couldhave a different composition than DOM exported during base-flow. Molecular analysis performed on samplescoming from the outlet of the Kervidy-Naizin catchment, an agricultural catchment located in France (CriticalZone Observatory AgrHyS) revealed a modification in the distribution of lignin compounds during flood events.This DOM, less biodegraded, could be produced by partition between particulate and dissolved phases when thesoil/water ratio is low, that is to say when soil particles are isolated in water. The evolution of DOM compositionduring storm events has been assumed to reflect a combination of in-stream and in-soil erosion processes. So howsoil erosion could be responsible for production of less degraded DOM? And is the composition of soil DOMmodified during a storm event?Those questions were investigated during two flood events, by sampling soil solutions with high frequency inriparian soils equipped with zero-tension lysimeters in the Kervidy-Naizin catchment. In the same time streamDOM was sampled at the outlet of the watershed and runoff were investigated. Samples have been filtered at0.2m, analysed for DOC and freeze-dried for molecular analysis (thermally assisted hydrolysis methylation - gaschromatography / mass spectrometry). The hydraulic gradient was monitored every 15 minutes using piezometersimplemented in the riparian soils and higher up in the toposequence.At the beginning of the events, hydraulic gradient increased rapidly and stayed high during several days. Modificationof DOM composition in soil solution were recorded during the hydraulic gradient rise with an increase inthe proportion of less degraded and more hydrophobic molecules than during base-flow conditions. Flood eventswere also responsible for contribution to the river of high quantity of runoff solutions containing a high amountof POM, and DOM in high quantity with the same characteristics than soil solutions. In river, the modification ofDOM quality is also recorded short time after its appearance in soil solutions.For the first time, the composition of soil solution DOM has been investigated during flood events. Resultsevidence the concomitant evolution of hydraulic gradient, DOM concentration and composition in runoff, soilssolutions and stream water. These results highlight the contribution of surface and sub-surface erosion in theproduction of storm stream DOM