Description: Salt marshes provide various ecosystem functions and services including flooding protection, wildlife habitats, and carbon storage. These functions and services could, however, be strongly impacted by anthropogenic activities such as livestock grazing – a common practice in the Wadden Sea salt marshes located in North of Germany. To assess the impact of grazing on soil parameters, a total number of eight soil cores (⌀: 18 cm; L: 50 cm) were collected in areas with and without livestock grazing, and scanned using a Computed Tomography (CT) to characterize soil parameters including soil macroporosity, sediment density, and pores connectivity. Subsequently, sub-samples were taken for determination of soil moisture content (%) and bulk density (g cm−3). To account for the impact of grazing on soil drainage after tidal inundations, water table relative to soil surface was monitored during two flooding events. Our results demonstrated that grazed salt marsh has higher top-soil bulk density, and lower macroporosity and pore connectivity, than ungrazed marsh, due to soil compaction by livestock grazing. Moreover, grazed marsh has slower water drainage and that might keep the soil waterlogged for a longer period of time which has implications on lowering decomposition rate due to lower soil redox. This study provides evidence that grazing alters physical soil parameters in salt marsh. Consequently, grazing needs to be accounted for when evaluating how land use impacts ecosystem services and functions including carbon sequestration.

Description: Root-aerenchyma in wetland plants facilitate transport of oxygen from aboveground sources (atmosphere and photosynthesis) to belowground roots and rhizomes, where oxygen can leak out and oxygenate the otherwise anoxic soils. In salt marshes, the soil oxygenation capacity varies among different Spartina-taxa, but little is known about structural pattern and connectivity of root-aerenchyma that facilitates this gas transport. Both environmental conditions and ploidy level play a role for the root-system morphology. Root-system morphology of polyploid Spartina-taxa was studied, quantifying root-tissue volume and root-aerenchyma volume of hexaploid Spartina alterniflora, Spartina maritima, and Spartina × townsendii as well as dodecaploid Spartina anglica from different habitats. Computed tomography (CT)-scan image analysis was applied to quantify the volume of roots and aerenchyma, and to determine the root-system structure (ratio of aerenchyma to root-tissue volumes) and aerenchyma connectivity. On average, Spartina-roots accounted for 12% (v/v) and root-aerenchyma accounted for 1% (v/v) of the soil volume in the pioneer marsh. About 90% (v/v) of all roots were associated with aerenchyma. Root-system structures of S. × townsendii and S. anglica differed and showed clear responses to habitat conditions, such as flooding regime and redox potential. The development of large well-connected aerenchyma fragments were specifically shown in S. anglica and to a minor extend in S. maritima. Aerenchyma in S. alterniflora and S. × townsendii consisted only of smaller fragments. Spartina-dominated tidal pioneer marsh soils show high connectivity with the atmosphere via root-aerenchyma. The high ploidy level in S. anglica comes along with high connectivity in root-aerenchyma.