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Tidal wetlands have been increasingly recognized as long-term carbon sinks in recent years. Work on carbon sequestration and decomposition processes in tidal wetlands focused so far mainly on effects of global-change factors such as sea-level rise and increasing temperatures. However, little is known about effects of land use, such as livestock grazing, on organic matter decomposition and ultimately carbon sequestration. The present work aims at understanding the mechanisms by which large herbivores can affect organic matter decomposition in tidal wetlands. This was achieved by studying both direct animal-microbe interactions and indirect animal-plant-microbe interactions in grazed and ungrazed areas of two long-term experimental field sites at the German North Sea coast. We assessed bacterial and fungal gene abundance using quantitative PCR, as well as the activity of microbial exo-enzymes by conducting fluorometric assays. We demonstrate that grazing can have a profound impact on the microbial community structure of tidal wetland soils, by consistently increasing the fungi-to-bacteria ratio by 38-42%, and therefore potentially exerts important control over carbon turnover and sequestration. The observed shift in the microbial community was primarily driven by organic matter source, with higher contributions of recalcitrant autochthonous (terrestrial) vs. easily degradable allochthonous (marine) sources in grazed areas favoring relative fungal abundance. We propose a novel and indirect form of animal-plant-microbe interaction: top-down control of aboveground vegetation structure determines the capacity of allochthonous organic matter trapping during flooding and thus the structure of the microbial community. Furthermore, our data provide the first evidence that grazing slows down microbial exo-enzyme activity and thus decomposition through changes in soil redox chemistry. Activities of enzymes involved in C cycling were reduced by 28-40%, while activities of enzymes involved in N cycling were not consistently affected by grazing. It remains unclear if this is a trampling-driven direct grazing effect, as hypothesized in earlier studies, or if the effect on redox chemistry is plant mediated and thus indirect. This study improves our process-level understanding of how grazing can affect the microbial ecology and biogeochemistry of semi-terrestrial ecosystems that may help explain and predict differences in C turnover and sequestration rates between grazed and ungrazed systems.

17Accretion rates, defined as the vertical growth of salt marshes measured in mm per 18 year, may be influenced by grazing livestock in two ways: directly, by increasing soil 19 compaction through trampling, and indirectly, by reducing aboveground biomass and thus 20 decreasing sediment deposition rates measured in g/m² per year . Although accretion rates 21 and the resulting surface elevation change largely determine the resilience of salt marshes to 22 sea-level rise (SLR), the effect of livestock grazing on accretion rates has been little studied. 23Therefore, this study aimed to investigate the effect of livestock grazing on salt-marsh 24 accretion rates. We hypothesise that accretion will be lower in grazed compared to ungrazed 25 M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 salt marshes. In four study sites along the mainland coast of the Wadden Sea (in the south-26 eastern North Sea), accretion rates, sediment deposition rates, and soil compaction of grazed 27 and ungrazed marshes were analysed using the 137 Cs radionuclide dating method. Accretion 28 rates were on average 11.6 mm yr -1 during recent decades and thus higher than current and 29 projected rates of SLR. Neither accretion nor sediment deposition rates were significantly 30 different between grazing treatments. Meanwhile, soil compaction was clearly affected by 31 grazing with significantly higher dry bulk density on grazed compared to ungrazed parts. 32Based on these results, we conclude that other factors influence whether grazing has an effect 33 on accretion and sediment deposition rates and that the effect of grazing on marsh growth Introduction 38Many coasts of the world show an enhanced rate of sea-level rise (SLR) over the past 39 century, and studies predict it to accelerate in the future (IPCC, 2007; Vermeer and 40 Rahmstorf, 2009). Global SLR was 3.1 mm yr -1 between 1993and 2003 (IPCC, 2007 unique flora and fauna (Schmidt et al., 2012). 54Given that lateral erosion is not occurring, the resilience of salt marshes to SLR is 55 largely determined by their ability to compensate higher water levels by increased vertical 56 accretion and/or reduced soil subsidence rates leading to increased surface elevation. Only if 57 accretion rates and the resulting increase in surface elevation are higher than rates of SLR, a 58 salt marsh will be able to keep pace with relative SLR. The surface elevation change in salt 59 marshes is the sum of sediment accretion, erosion, compaction processes, and possibleregional crustal movements (French, 1993 French et al., 2003). Many studies have investigated accretion rates in salt marshes (e. g. 74Cahoon and Turner, 1989; Dijkema, et al. 1990; Dijkema, 1997; Bellucci et al., 2007; 75 Baustian et al., 2012), and several models exists to predict the future development of salt 76 marshes (e.g. Allen, 1990;Temmerman et al., 2003; Bartholdy et al., 2004; French, 2006, 77 Schuerch et al., 2013). Yet, the question of whether accretion rates and the resulting surface 78 elevat...

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