Impact of water-table dynamics on the destabilization of soil organic matter in a temperate agricultural catchment

International audienceSoil organic matter (SOM) destabilization produces gaseous and dissolved species and plays a key-role toward environmental and climatic issues. This process can be viewed as a combination of abiotic (solubilization) and biotic (biodegradation) processes. Both of them are controlled by the water occurring in soils since this molecule is at the same time the solvent and the living media. The soil water content has been shown to control the emission of soil carbon dioxide and the water-table dynamic to control the temporal variations of dissolved organic carbon in soil solutions. The aim of this study is to explore how the fluctuations of water-table levels can impact the gaseous and dissolved components of SOM destabilization. This was performed on the Kervidy-Naizin catchment (AgrHys critical zone observatory) localized in Brittany and that is part of the OZCAR Research Infrastructure. Measurments of CO 2 fluxes and soil solution species (dissolved organic and inorganic carbon DOC/DIC and ferrous cation FeII as a) sampled in the macroporosity were performed fortnightly during the hydrologic year 2014-2015 from October to May at two sampling sites. They were chosen along a topographic transect equipped to monitor water-table levels with one downslope (Down) and one midslope (Mid). The difference in water-table fluctuations were measured by the mean residence time and mean return time. Down the residence and return times were 47 and 4 days, respectively, while Mid they were 2 and 12 days, respectively. Along the sampling period, the flux of CO 2 was higher at Mid (1.5 ± 0.5 µmol m-2 s-1 ; mean ± standard deviation) than at Down (1.0 ± 1.0 µmol m-2 s-1). At both sites CO 2 decreased from October to February and then increased up to May, however the kinetics were different. DIC and DOC were differently correlated in the two sites. At Mid, they were inversely correlated, with a steady DOC decrease from 14 to 8 mg/l from October to March and a steady DIC increase from 9 mg/l to 30 mg/l. At Down two periods were defined by the occurrence of FeII that is a marker of reducing conditions. Before the reducing period DIC and CO 2 were inversely correlated, while during reducing conditions DIC, DOC and CO 2 were positively correlated and increased by 4, 3 and 8 respectively, highlighting reducing period as a hot moments for soil organic matter biodegradation in lowland wetlands. The results of this study, together with literature survey, are used to propose a conceptual model of SOM destabilization