Mimicking floodplain reconnection and disconnection using 15N mesocosm incubations

Floodplain restoration changes the nitrate delivery pattern and dissolved organic matter pool in backwaters, though the effects these changes have are not yet well known. We performed two mesocosm experiments on floodplain sediments to quantify the nitrate metabolism in two types of floodplains. Rates of denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and anammox were measured using 15N-NO tracer additions in mesocosms of undisturbed floodplain sediments originating from (1) restored and (2) disconnected sites in the Alluvial Zone National Park on the Danube River downstream of Vienna, Austria. DNRA rates were an order of magnitude lower than denitrification and neither rate was affected by changes in nitrate delivery pattern or organic matter quality. Anammox was not detected at any of the sites. Denitrification was out-competed by assimilation, which was estimated to use up to 70% of the available nitrate. Overall, denitrification was higher in the restored sites, with mean rates of 5.7 ± 2.8 N m-2 h-1 compared to the disconnected site (0.6 ± 0.5 N m-2 h). In addition, ratios of NO : N were lower in the restored site indicating a more complete denitrification. Nitrate addition had neither an effect on denitrification, nor on the NO : N ratio. However, DOM (dissolved organic matter) quality significantly changed the N O : N ratio in both sites. Addition of riverine-derived organic matter lowered the NO : N ratio in the disconnected site, whereas addition of floodplain-derived organic matter increased the NO : N ratio in the restored site. These results demonstrate that increasing floodplains hydrological connection to the main river channel increases nitrogen retention and decreases nitrous oxide emissions