Nitrogen and Vegetation Dynamics in European Salt Marshes

Abstract

In this paper we discuss the role of nitrogen in the vegation dynamics of European salt marshes. An overview is presented of various fluxes of nitrogen to and from salt marsh ecosystems. Our primary view is on European salt marshes. A nitrogen budget constructed for salt marsh ecosystems in the Wadden Sea area shows that N-influx rates via floodwater (50–200 kg N ha−1 y−1) and via atmospheric deposition (30 kg N ha−1 y−1), the rate of N assimilation (95–230 kg N ha−1 y−1) and the rate of mineralization (28–225 kg N ha−1 y−1) vary and fluctuate in space and time but are of the same order of magnitude. These N-influx and efflux rates may differ markedly between salt marshes and within salt marshes. Within salt marshes the in and effluxes vary along elevational gradients with varying flooding frequencies and sedimentation rates and along successional gradients. Salt marshes are productive and dynamic ecosystems with variable and extreme environmental conditions. Rather than acting as a source or sink for nitrogen the transformations of organic and inorganic nitrogen tend to characterize salt marsh ecosystems. The cycling of N through a sediment-plant-sea water-atmosphere salt marsh ecosystem is also discussed. In addition, the impact of coastal salt marshes in reducing levels of inorganic N from sea water is assessed, 15N-stable isotope studies indicate that, despite the significant influxes of N via atmospheric deposition and seawater flooding to the salt marsh ecosystems, most inorganic N is taken up via the root system and not through foliar uptake. There is strong evidence that availability of nitrogen limits plant growth and vegetation succession in salt marshes. It is hypothesized that increased input of nitrogen via floodwaters (polluted rivers and estuaries) and atmosphericdeposition (agriculture, industry, traffic) has affected the development of salt marsh vegetation. In particular the increasing dominance of the grass Elymus athericus during the last decades is related to the increased supply of nutrients to salt marshes. The impact of eutrophication via seawater and atmospheric deposition is discussed in relation to changes in species composition of various vegetation zones and successional stages in grazed and ungrazed salt marshes. Analysing changes in species composition over periods of years and decades, longer term changes of other environmental factors such as sedimentation, flooding frequency and flooding duration and nitrogen content of the salt marsh soil are also surveyed. The increased occurrence of Elymus athericus in West European salt marshes may relate to eutrophication. Alternatively, the dominance of the tall and rapidly growing Elymus athericus is linked with the “naturally” enhanced N-soil content and increased rate of N-mineralization in late successional stages of salt marshes.