Soil developments in salt marshes and on artificial islands in the Wadden Sea

Salt marshes are located at the interface between marine and terrestrial ecosystems and inhabited by a unique and highly adapted flora and fauna. Requiring a moderate tidal range, salt marshes are characterised by a typical zonation: The pioneer zone represents the first elevation step above the tidal flat being daily flooded. It is followed by the lower salt marsh zone, as well as the upper salt marsh zone which is flooded only several times a year. Because of this wide range in inundation frequency, zones can be distinguished into a proximal area favoured by specialists and a competition-based environment in the areas more distal to the sea. While in the former, biota must cope with waterlogged, anoxic and saline soils, in the latter, plants compete for scarce nutrients in often depleted soils. This thesis deepens the understanding of the interplay of abiotic factors in the salt marsh system induced by regular inundation and its consequences for carbon balances, basal environmental functions such as decomposition, as well as adaptations of biota to these stressful conditions. In chapter 2 of this thesis, soils from the pioneer- and lower salt marsh zone were investigated concerning the turnover of soil organic matter (SOM) under artificially altered tidal inundation cycles in a laboratory setup. One focus was the priming effect (PE), describing short-term changes in the SOM-turnover induced by the input of certain substances or mechanical treatments of the soil. Moreover, the composition of the prokaryotic soil community accompanying the treatments of “all-time ebb”, “all-time flooded” and (every eight hours) “changing water level (Tide)” was analysed. The aim was to investigate changes in the CO 2 -efflux, PE and community composition resulting from different soil sampling locations within the salt marsh and changed inundation cycles. Samples from the higher elevation showed higher CO 2 -efflux rates vs. samples from the low elevation due to higher SOM content. Cumulative CO 2 -efflux was highest in the “Tide”-treatment, whereas PE could only be verified under “all-time ebb” conditions. While cumulative CO 2 -results can be explained by oxygen conditions typical for these soils, PE was affected by changes in prokaryotic metabolism. With respect to prokaryotic community composition, evidence for temporal niche adaptation was found under applied water level treatments. Chapter 3 represents a field study focussing on decomposition, a basal ecosystem service. Artificially constructed islands in the Wadden Sea were used as models for disturbed salt marsh systems. As a main hypothesis, we expected a slower decomposition activity on disturbed systems due to decomposer biodiversity loss. Within a two-week period, loss of biomass of an easily available substrate, decomposer diversity and soil nutrient status was recorded. Biomass loss was different between the salt marsh zones only on the reference plots, but not on the islands, with the most rapid loss in the upper salt marsh zone. Decomposer diversity correlated with salt marsh elevation. Carbon-to-nitrogen-ratio (C/N) declined with biomass loss due to protein enrichment of the topsoil during decomposition. Moreover, results showed a clear negative correlation between the C/N-ratio and species richness, leading to the conclusion of a higher decomposition rate and nutrient deliverance due to higher decomposer diversity. In chapter 4, we examined the effects of temperature, sea level and coastal eutrophication on decomposition and stabilisation of SOM in tidal wetlands on a global scale. To achieve comparable results, we used two types of standardised litterbags and determined their initial weight and the biomass loss after a period of ~90 days during which they were buried in the topsoil of mangrove and salt marsh systems. Whereas decomposition constant k did not show significant results, stabilisation of SOM (S) was negatively affected by higher mean temperatures and under more frequent inundation cycles. Chapter 5 focussed on root traits of various salt marsh plants in conjunction with typical parameters of stress occurring in salt marsh ecosystems such as inundation, high salinity or anoxic soils. The main hypothesis was that sediment features, from oxygen status to nutrient status, shape root traits of salt marsh plants and that pioneer plants show a superior adaptation to tidal inundations which manifests in root-mass, -length, -area and -tissue density. As one root trait, fine root mass was highest in the lower salt marsh zone, indicating competition for nutrients in this zone of highest biodiversity. Fine root surface area was negatively correlated to nutrient load of the soil showing adaptations of plants to the nutrient-poor sites. Overall, I could show that tidal inundation is the key factor in salt marsh ecosystems which determines SOM, PE, decomposition as an ecosystem function and root traits. Regarding C-balances, inundation can lead to decreased CO 2 -efflux via decreased gas diffusion and decreased soil C sequestration via improved moisture supply. PE seems to be less expressed under flooded conditions due to differences in prokaryotic metabolisms compared to terrestrial systems. Using decomposition and root traits, this thesis shows that flooding frequency shapes local biodiversity resulting in shifts of ecosystem functions