Nitrogen processing in a tidal freshwater marsh: a whole-ecosystem [sup.15]N labeling study

We quantified the fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrientrich Scheldt River in a whole-ecosystem [sup.15]N labeling experiment. [sup.15]N-N[H.sup.4.sub.4] was added to the floodwater entering a 3,477 [m.sup.2] 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 N[O.sup.-.sub.3] N[O.sup.-.sub.2], [N.sub.2]O, [N.sub.2] N[H.sup.4.sub.4], 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 budget was almost closed (115% recovery). All analyzed dissolved and suspended N pools were labeled, and 31% of added [sup.15]N-N[H.sup.+.sub.4] was retained or transformed. Nitrate was the most important pool for [sup.15]N, with nitrification accounting for 30% of [sup.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 [N.sub.2]O. Our results show that tidal freshwater marshes function not only as nutrient sinks but also as nutrient transformers.