Effect of decomposing litter on the mobility and availability of metals in the soil of a recently created floodplain

Some newly created wetland areas in the Scheldt estuary are heavily contaminated by metals. They are expected to be colonised by reed (Phragmites australis) and, on a longer term, willow (Salix ssp.). Supplying litter or stimulating plant biomass production on the short term could be possible management options to restrict metal mobility or availability in the upper soil layer. The influence of litter application on the mobility and availability of metals in the top layer of a soil of a recently created floodplain along the river Scheldt (Schelde) was studied in a greenhouse experiment. Reed stem, reed leaf and willow leaf litter were dried, ground and added to the soil. The treated soil was subsequently subjected to permanently flooded and alternately flooded/drained conditions. Metal concentrations were measured in the pore water, surface water above the soil and percolates. Moreover, reed plants (P. australis), duckweed (Lemna minor) and willows (Salix cinerea) were allowed to grow on these soils. Metal concentrations in the plants, growth and chlorophyll fluorescence were measured. Decomposing litter was found to affect the metal mobility and availability. During the first weeks after litter supply, the concentrations of some metals (e.g. Cd, Cu, Ni and Zn) in the pore water markedly exceeded remediation thresholds for ground water. The metal concentrations started to decrease after some days or weeks, which might be attributed to re-adsorption to different soil phases, a decreasing presence of soluble organometallic complexes and a lowering competition with Ca2+ for sorption to the solid soil phase. However, most data point towards a role of sulphide precipitation upon sulphate reduction, which is affected by the type of litter supplied to the soil. The used willow leaf material was found to release more sulphates compared to the litter originating from reed plants. When the soils were alternately flooded, metals were exported through the percolates. Total metal export substantially increased when litter was added. Growth and metal concentrations in duckweed, reed plants and willows were affected by the supply of litter, whereas photosynthesis was less affected. The experiments should still be performed on pilot scale before developing wetland management guidelines from these results. (C) 2008 Elsevier B.V. All rights reserved.