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.

Blue carbon ecosystems, including salt marshes, play an important role in the global carbon cycle because of their high efficiency to store soil organic carbon (OC). Few studies focus on the origin of OC stored in salt-marsh soils, which comes from either allochthonous or autochthonous sources. The origin, however, has important implications for carbon crediting approaches because the alternative fate of allochthonous OC (AllOC), i.e. if it had not accumulated in the Blue C ecosystem, is unclear. Here, we assessed the origin of OC in two mainland salt-marsh sites of the European Wadden Sea, analyzing δ 13 C of topsoil (0-5 cm) samples, freshly deposited sediment (allochthonous source), and of above-and belowground biomass of vegetation (autochthonous sources). We tested for effects of geomorphological factors, including elevation and the distance to sediment sources, and of livestock grazing, as the most important land-use form, on the relative contributions of allochthonous versus autochthonous sources to the topsoil OC stock. A negative effect of distance to the creek on the relative contribution of AllOC was found at only one of the two salt marshes, probably 2 due to differences in micro-topography between the two salt marshes. Additionally, the relative contribution of AllOC increased with increasing distance to the marsh edge in areas without livestock-grazing, while it decreased in grazed areas. Our findings demonstrate that spatial factors such as surface elevation and distance to a sediment source, which have been found to determine the spatial patterns of sediment deposition, also are important factors determining the relative contribution of AllOC to topsoil OC stocks of salt marshes. Furthermore, we provide first evidence that livestock-grazing can reduce the relative contribution of AllOC to the soil OC stock. These findings thereby yield important implications for C crediting and land-use management.