Nitrogen processing in a tidal freshwater marsh: A whole ecosystem super(15)N labeling study

• Published in: Limnology and oceanography 2005-11, Vol.50 (6), p.1945-1959
• Main Authors: Gribsholt, Britta, Boschker, Henricus TS, Struyf, Eric, Andersson, Maria, Tramper, Anton, De Brabandere, Loreto, Van Damme, Stefan, Brion, Natacha, Meire, Patrick, Dehairs, Frank, Middelburg, Jack J, Heip, Carlo HR
• Format: Article
• Language: English
• Subjects: Freshwater
• ISSN: 0024-3590; 1939-5590

We quantified the fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River in a whole-ecosystem super(15)N labeling experiment. super(15)N-NH 4 super(+) was added to the floodwater entering a 3,477 14 m2 tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. We present data for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of NO 3 super(-), NO 2 super(-), N 2 O, N 2 , NH 4 super(+), and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. Simultaneous addition of a conservative tracer (NaBr) confirmed that tracer was evenly distributed, and the Br super(2) budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added super(15)N-NH 4 super(+) was retained or transformed. Nitrate was the most important pool for super(15)N, with nitrification accounting for 30% of super(15)N-transformation. In situ whole-ecosystem nitrification rates were four to nine times higher than those in the water column alone, implying a crucial role for the large reactive marsh surface area in N-transformation. Under conditions of low oxygen concentrations and high ammonium availability, nitrifiers produced N2O. Our results show that tidal freshwater marshes function not only as nutrient sinks but also as nutrient transformers.