River-floodplain ecosystems are long-term sinks for certain contaminants. Fluctuations between flooding and drying are major drivers of spatial and temporal changing biogeochemical processes within floodplain soils. Even small changes of the frequency, magnitude, and timing of drying and rewetting can episodically turn those ecosystems into a source for matter.

To understand such changes we investigated the effects on top soils of three hydro-geomorphological units that are typical for floodplains: depression, levee, and plateau. They show distinct characteristics regarding soil type, trace element (TE) pollution level and hydrology. In the laboratory we used an automated biogeochemical microcosm set up to simulate oxidized–reduced cycles to monitor the effects of strong fluctuating redox potential (E_H), on pH, iron (Fe), manganese (Mn), sulfur (S), nitrate (NO₃), dissolved organic carbon (DOC) and dissolved concentrations of arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), nickel (Ni), and zinc (Zn) in the soil solution.

The results indicated different responses regarding the course of E_H/pH and different release patterns of TEs for each hydro-geomorphological unit separately. Both could be mainly allocated to their explicit spatial characteristics; due to differences in its buffer capacities and to the interaction between pH-value and microbial activity. In particular, we could demonstrate that a combination of several biogeochemical factors may influence TE release or retention in floodplain soils by means of generalized additive models (GAMs). Using those, Fe and S were identified as most important factors that determined the dynamics of TEs in all three studied soils; concentrations of As, Cu, Cr, Ni, and Zn increased with increasing Fe-concentrations whereas Cd, Cu, Cr, Ni, and Zn seem correspondingly to be dependent on S that went along with the course of the oxidized–reduced cycle.

Still, several uncertainties between environmental changes and TE release from floodplain soils may exist. Thus, the fate of dissolved TEs in river-floodplain ecosystems needs to be taken under serious consideration since the ecotoxicology threat of them is of undisputed risk.