Spatial and temporal variations of dissolved organic matter dynamics in adisturbed Sphagnum peatland after hydrological restoration

International audiencePeatlands contain a third of the world soil C in spite of their relatively low global area (3% of land area). They canbecome sources of C because of human disturbances such as drainage. The aim of this work is to assess the effectof an hydrological restoration on a disturbedSphagnumpeatland. It concerns spatial and temporal variations ofdissolved organic matter (DOM) dynamics.The investigated site was La Guette peatland (France, N 47◦19’44”, E 2◦17’04”, alt. 154m), whose hydrologicalconditions are influenced by a road passing through its former area. The road drain accelerates drying mechanisms,favouring thus vascular plants settlement to the detriment of specific flora of peatlands (i.e.Sphagnum). Hydro-logical restoration was undertaken in February 2014. It consisted in building thresholds to slow down drain runoffand to promote the soil rewetting. Two transects of piezometers were settled in independent two hydrologicalsub-systems: Trans-up and Trans-down. Trans-down is supposed to be influenced by the hydrological restoration,while Trans-up is not. These transects cross the peatland and follow water flow direction until the outlet. Sixsampling campaigns were performed before, during and after the vegetation period. Water conductivity and pHwere measured on site. Water samples were collected in the piezometers. Samples were filtered in the field at0.45μm. Concentrations of dissolved organic carbon (DOC), cations (Na+, K+, Ca2+, Mg2+, NH4+) and anions(Cl−, SO42−, PO43−, NO2−, NO3−) were measured. Absorbance was measured by UV-VIS spectrophotometerto assess SUVA254and aromaticity of DOM. Three-dimensional excitation–emission matrices (EEM) wereundertaken to characterise fluorescent DOM (FDOM). Humification (HIX) and biological (BIX) fluorescenceindices were calculated. PARAFAC algorithm was used to treat EEMs. Precipitations and water levels weremeasured automatically by a weather station and automatic probes, respectively. Rain water was also analysed toassess precipitation contribution in each analysis.Mean DOC concentrations are higher in Trans-up than in Trans-down (45 mg.L−1vs30 mg.L−1). Watertable fluctuations are more important in Trans-up than in Trans-down (6 cmvs15 cm). In both sub-systems,DOC concentrations decrease from the upstream forest border to the middle of the peatland and then increaseuntil the outlet. DOM aromaticity shows seasonal variability with a peak in summer. Maximum aromaticity isalways reached in the middle of transects. Measured emission/excitation couples are similar to those found inpeat standards and references (International Humic Substances Society). Fluorescence indices show that DOMhumification degree increases while crossing peatland following water flow direction. Seasonal variations offluorescent intensity ratios are wider in Trans-up than in Trans-down due to higher water drawdown duringsummer in Trans-up. DOM in the middle of the transects is mainly autochthonous, whereas DOM near the limit ofthe peatland shows strong allochthonous influence from neighbour systems. Ionic concentrations are correlated toDOC, showing charging and discharging gradients in peat water. Results from characterisation of DOM and peatwater geochemistry show spatial and temporal variations in nature and origins of DOM. This study emphasizescontrasts between control and hydrologically reworked sub-systems