Ammonium Transformation in a Nitrogen-Rich Tidal Freshwater Marsh

Abstract

The fate and transport of watershed- derived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River, Belgium, was quantified in a whole ecosystem 15 N labeling experiment. In late summer (September) we added 15 N-NH4 to the flood water entering a 3477 m 2 tidal freshwater marsh area, and traced the ammonium processing and retention in four sub- sequent tide cycles. In this paper we present the results for the water-phase components of the marsh system and compare them to a similar experiment conducted in spring/early summer (May). Changes in concentration and isotopic enrichment of NO3 +N O 2 ,N 2O, N2 ,N H 4 and suspended particulate nitrogen (SPN) were mea- sured in concert with a mass balance study. All analyzed N-pools were labeled, and 49% of the added 15 NH4 was retained or transformed. The most important pool for 15 N was nitrate, account- ing for 17% of 15 N-transformation. N2 ,N 2O and SPN accounted for 2.4, 0.02 and 1.4%, respectively. The temporal and spatial patterns of 15 N transfor- mation in the water phase component of the system were remarkably similar to those observed in May, indicating good reproducibility of the whole eco- system labeling approach, but the absolute ammonium transformation rate was 3 times higher in May. While the marsh surface area was crucial for nitrification in May this was less pronounced in September. Denitrification, on the other hand, appeared more important in September compared to May.