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 Environmental Engineering ETH Zurich, Schaffmattstrasse 6, CH-8093, Switzerland
4 INRA, 17 rue Sully, BP 86510, FR-21000 Dijon, France
5 Institute of Agricultural Sciences, ETH Zurich, Eschikon 33, CH-8315 Lindau, Switzerland
6 Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), and Institute of Biology, Freie Universität Berlin, Müggelseedamm 310, DE-12587 Berlin, Germany
Flood pulses are major drivers of river-floodplain processes. We investigated their effects on soil nitrogen (N) transformations along a lateral gradient from the river to a mature alluvial forest in the Thur River floodplain (NE Switzerland). Selected N pools (ammonium and nitrate) and N transformations (mineralization, nitrification and denitrification) in the topsoils were repeatedly quantified over a period of six months. During this period, the floodplain was affected by two flood events of different magnitude.
Our results showed a distinct difference in size and temporal changes of N pools and transformation rates among functional process zones (FPZs) differing in flooding disturbance. A strong temporary increase in N mineralization in a FPZ with young soil on the frequently flooded gravel bars was the most prominent flood-related effect. This was most likely related to the fast-flowing water during inundation that deposited fresh dispersed sediments with a coarse sandy texture containing highly bioavailable organic N, and/or stimulated the turnover of existing N by destroying sediment aggregates already present before the flood. Increased N mineralization appeared to stimulate coupled nitrification-denitrification in this FPZ during the drying phase. In the more stable FPZs, N mineralization was not strongly enhanced by flooding, whereas nitrification and denitrification were also increased during the drying phase, however to a lesser degree than in the gravel bar soils. Based on our results we propose that floodplain zones characterized by short intensive floods with fast over-flowing water are 'hot spots' and the drying phases after the floods are 'hot moments' of N transformations.
Keywords: nitrogen mineralization, nitrification, denitrification, flood pulse, floodplain soils