N and Si cycling in freshwater tidal marshes

Tidal marshes have high sediment surface areas, promoting processes affecting nutrient speciation, transformation and retention. In this project we performed whole-ecosystem 15NH4 + tracer addition experiments to quantify the fate and transport of ammonium through a tidal freshwater marsh. Combined with a mass-balance study, this approach allowed a simultaneous examination of transport and processing of nitrogen. We also performed additional studies on Si cycling in parallel to the N-labeling experiment. Our work shows that the large reactive surface of the tidal freshwater marsh vegetation is crucial for nitrogen transformation and assimilation. It clearly revealed the dominant role of microbes in initial nitrogen retention in marsh ecosystems. Parallel study of silica cycling revealed that tidal marshes act as biogenic Si recycling surfaces, importing biogenic Si while exporting dissolved Si. Export of dissolved Si is greatest during summer and spring, when dissolved Si concentrations in inundating waters are depleted by diatoms. Tidal marshes thus buffer estuarine dissolved Si in times of limitation. We can conclude that the studied marsh had an increasing effect on the Si:N ratio in flooding waters. Export of DSi and import of total dissolved nitrogen (DIN) contributed about equally to the increase of the Si:N ratio