Mimicking floodplain reconnection and disconnection using 15N mesocosm incubations

[摘要] Floodplain restoration changes the nitrate delivery pattern and dissolvedorganic matter pool in backwaters, though the effects these changes have arenot yet well known. We performed two mesocosm experiments on floodplainsediments 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-NO₃ tracer additions inmesocosms of undisturbed floodplain sediments originating from (1) restoredand (2) disconnected sites in the Alluvial Zone National Park on the DanubeRiver downstream of Vienna, Austria. DNRA rates were an order of magnitudelower than denitrification and neither rate was affected by changes innitrate delivery pattern or organic matter quality. Anammox was not detectedat any of the sites. Denitrification was out-competed by assimilation, whichwas estimated to use up to 70% of the available nitrate. Overall,denitrification was higher in the restored sites, with mean rates of5.7 ±2.8 mmol N m⁻² h⁻¹ compared to the disconnected site(0.6 ±0.5 mmol N m⁻² h⁻¹). In addition, ratios ofN₂O : N₂ were lower in the restored site indicating a more completedenitrification. Nitrate addition had neither an effect ondenitrification, nor on the N₂O : N₂ ratio. However, DOM (dissolved organic matter) qualitysignificantly 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-derivedorganic matter increased the N₂O : N₂ ratio in the restored site.These results demonstrate that increasing floodplains hydrologicalconnection to the main river channel increases nitrogen retention anddecreases nitrous oxide emissions.

[学科分类] 地球化学与岩石