Flood pulses control soil nitrogen cycling in a dynamic river floodplain

Abstract 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.