Flood pulses control soil nitrogen dynamics in a dynamic river floodplain

Shrestha Juna 1, Niklaus Pascal A. 2, Pasquale Nicola 3, Huber Benjamin 1, Barnard Romain L. 4, Frossard Emmanuel 5, Schleppi Patrick 1, Tockner Klement 6, Luster Jörg 1

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

Geoderma 228-229 (2014): 14-24

DOI: 10.1016/j.geoderma.2013.09.018


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