Nitrogen assimilation and short term retention in a nutrient-rich tidal freshwater marsh ? a whole ecosystem 15N enrichment study
Résumé
We conducted two (May 2002 and September 2003) pulse additions of 15NH4+ to the flood water inundating a tidal freshwater marsh fringing the nutrient-rich Scheldt River (Belgium) and traced the fate of ammonium in the intact ecosystem. Here we report in detail the 15N uptake into the various marsh components (leaves, roots, sediment, leaf litter and invertebrate fauna), and the 15N retention on a scale of 15 days. We particularly focus on the contributions of the rooted macrophytes and the microbial community in the sediment and on plant litter. Assimilation and short term retention of 15NH4+ was low on both occasions. Only 4?9% of the added 15N trace was assimilated, corresponding to 13?22% and 8?18% of the processed 15N (i.e. not exported as 15NH4+ in May and September, respectively. In May nitrogen assimilation rate (per hour inundated) was >3 times faster than in September. Macrophytes (above- and below ground) were of limited importance for short term 15N retention accounting for <6% of the total 15NH4+ processed by the marsh. The less dominant herbaceous species were more important (on an area basis) than the dominant reed (Phragmites australis). The microbial community colonizing the sediment and litter surfaces were responsible for most nitrogen assimilation and short-term retention in the marsh. The large reactive surface area available for microbial colonization together with direct plant uptake, are the crucial components for nitrogen assimilation, retention and transformation in nutrient-rich tidal freshwater marshes.
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