Mimicking floodplain reconnection and disconnection using 15N mesocosm incubations

Abstract : Floodplain restoration changes the nitrate delivery pattern and dissolved organic matter pool in backwaters but other effects are not yet well known. We performed two mesocosm experiments 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 tracer additions in mesocosms containing 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 mmolNm−2 h−1compared to the disconnected site (0.6±0.5 mmolNm−1 h−1). In addition, ratios of N2O:N2 were lower in the restored site indicating a more complete denitrification. Nitrate addition did not have any effect on denitrification, nor on the N2O:N2 ratio. However, DOM quality significantly changed the N2O:N2 ratio in both sites. Addition of riverine derived organic matter lowered the N2O:N2 ratio in the disconnected site, whereas addition of floodplain derived organic matter increased the N2O:N2 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.
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Submitted on : Monday, October 29, 2012 - 4:53:28 PM
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N Welti, E Bondar-Kunze, M Mair, Patricia Bonin, W Wanek, et al.. Mimicking floodplain reconnection and disconnection using 15N mesocosm incubations. Biogeosciences Discussions, European Geosciences Union, 2012, 9, pp.4133-4176. ⟨10.5194/bgd-9-4133-2012⟩. ⟨hal-00746799⟩

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