1 Swiss Federal Institute for Forest, Snow and Landscape Research, Zürcherstr. 111, CH-8903 Birmensdorf, Switzerland
2 Institute of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland
3 Institute of Agricultural Sciences, ETH Zurich, Eschikon 33, CH-8315 Lindau, Switzerland
4 Institute of Biology, University of Neuchâtel, Rue Emile-Argand 11, CH-2000 Neuchâtel, Switzerland
5 Institute of Agricultural Sciences, ETH Zurich, Universitätstr. 2, CH-8092 Zurich, Switzerland
6 Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), and Institute of Biology, Freie Universität Berlin, Müggelseedamm 310, DE-12587 Berlin, Germany
In their natural state, river floodplains are heterogeneous and dynamic ecosystems that may retain and remove large quantities of nitrogen from surface waters. We compared the soil nitrogen dynamics in different types of habitat patches in a restored and a channelized section of a Thur River floodplain (NE Switzerland). Our objective was to relate the spatiotemporal variability of selected nitrogen pools (ammonium, nitrate, microbial nitrogen), nitrogen transformations (mineralisation, nitrification, denitrification) and gaseous nitrogen emission (N2O) to soil properties and hydrological processes. Our study showed that soil water content and carbon availability, which depend on sedimentation and inundation dynamics, were the key factors controlling nitrogen pools and processes. High nitrogen turnover rates were measured on gravel bars, characterized by both frequent inundation and high sediment deposition rates, as well as in low-lying alluvial forest patches with a fine-textured, nutrient-rich soil where anaerobic microsites probably facilitated coupled nitrification-denitrification. In contrast, soils in the dam foreland of the channelized section had comparatively small inorganic nitrogen pools and low transformation rates, particularly those related to nitrate production. To conclude, environmental heterogeneity characteristic of the restored section, favors nitrogen removal by creating locations of high sedimentation and denitrification. Of concern, however, are the locally high N2O efflux and the possibility that nitrate might leach from nitrification hotspots.