Mobilization of DOM during flood events: impact of water table dynamics
Résumé
dissolved organic matter (DOM) over hydrological year. During
these hot moments, the increased flow at the outlet is accompanied
with an increase of DOM concentration and a modification of
DOM composition, implying the mobilization of additional DOM
sources regard to base-flow conditions. Molecular analysis have
been performed on outlet samples of a French agricultural catchment
(Kervidy-Naizin) which belongs to the Critical Zone Observatory
AgrHyS (SOERE Réseau des Bassins Versants). Previous
results shown a clear modification in the distribution of lignin
compounds during flood events and evidence an heterogeneity in
the intensity of molecular changes. This DOM, less biodegraded,
was assumed to be produced by partition between particulate and
dissolved phases when the soil/water ratio is low (soil particles
isolated in water). However, in the outlet, modifications of DOM
composition is still recorded even after the return to pre-event
turbidity. To explain these results we hypothesize that molecular
changes could come from DOM produced in soil solutions. So how
soil erosion could induce production of less biodegraded DOM? Is
the intensity of molecular composition changes impacted by intensity
of the flood event?
Those questions were investigated during flood events, by sampling
with high frequency stream DOM at the outlet of the watershed
and soil solutions in riparian soils equipped with zero-tension
lysimeters. Samples were filtered, analyzed for DOC and freezedried
for molecular analysis (thermally assisted hydrolysis and
methylation–gas chromatography/mass spectrometry). The hydraulic
gradient was monitored every 15 minutes, using piezometers
implemented along the slope. The long-term monitoring program
of ORE AgrHyS on this site allowed a good understanding
of hydrological functioning of this catchment, which is essential
to link hydrology and biogeochemistry.
At the beginning of the event, hydraulic gradient increased rapidly
and stayed high during several days. Modification of DOM composition
in soil solutions were recorded during the hydraulic gradient
rise with an increase in the proportion of less degraded molecules
than during base-flow conditions. In river, modification of DOM
quality is also recorded short time after its appearance in soil solutions.
Intensity of the flood events seems also to impact intensity
of the modification with intense flood events responsible for high
variations in DOM composition.
For the first time, composition of soil DOM has been investigated
during flood events. Results evidence the concomitant evolution
of hydraulic gradient, DOM concentration and composition in soils
solutions and stream outlet. Results highlight the contribution of
sub-surface erosion in the production of storm stream DOM, and
the key role of hydrological conditions in these modifications