Use of dredged sediments for soil creation in the Seine estuary (France): Importance of a soil functioning survey to assess the success of wetland restoration in floodplains

The creation and restoration of new wetlands to mitigate wetland losses is a newly developing science whose success still needs to be assessed. This study focuses on the ecological restoration of a gravel-pit in the low valley of the Seine estuary (France). Restoration consisted in filling the gravel-pit using a hydraulic technique with dredged sediments from the Seine river and covering it with alkaline peat from adjacent wet meadows. Our objectives were to survey the functions of recreated soil 3 years after the gravel-pit was filled and assess whether it regained typical wetland functionality and to determine which soil functioning parameters are the most efficient for assessing restoration success. To address these questions, an approach combining analyses of in situ and ex situ soil functioning was used. The survey was conducted on recreated soil as compared to a control soil (i.e. soil before gravel extraction). Four topographic zones were sampled corresponding to 4 types of recreated soil functioning in terms of waterlogging conditions: Hemic Histosol without waterlogged periods, Hemic Histosol with temporary waterlogged periods, Hemic Histosol with the longest waterlogged periods and Interstratified Histosol without waterlogged periods. Soil respiration and SIR results showed that large stocks of organic matter are maintained after 3 years of restoration and proved able to sequester C in recreated soils. 3 years after restoration, nitrogen removal function measured through denitrification technique was restored in the Hemic Histosol with the longest waterlogged periods. These results demonstrate that waterlogging regime maintain the C stock and accelerate the restoration of denitrification process.