Origin of organic carbon in the topsoil of Wadden Sea salt marshes

Abstract

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.

Ungrazed salt marsh has well connected soil pores and less dense sediment compared with grazed salt marsh: a CT scanning study

“…1A). The area is characterized by a long history of human interventions, such as the construction of ditched sedimentation fields with brushwood groynes to enhance sedimentation (Mueller et al, 2019a) and high stocking density grazing (Esselink et al, 2009). The establishment of the National Park in J o u r n a l P r e -p r o o f ecosystem, long-term grazing experiments were installed in the late 1980s (Kiehl et al, 1996).…”

“…6) is lower with grazing, particularly, in the top profile of the soil cores. As a result of mechanical stresses that J o u r n a l P r e -p r o o f have been added to the soil surface, grazing can alter the physical and chemical nature of the soil, and that may negatively impact the ecosystems services and functions provided by saltmarshes including blue C storage (Davidson et al, 2017;Mueller et al, 2019a), however some studies reported that the impact of grazing on blue C storage is minimal on a broader-scale (Harvey et al, 2019).…”

Elymus athericus encroachment in Wadden Sea salt marshes is driven by surface elevation change

“…No data on the N content of freshly deposited sediment is available for this specific site, but assuming similar values to those of Mueller et al. (), the input of N via sediment deposition would range between 2.8 and 7.0 g m −2 year −1 . This illustrates that natural N input in these island marshes is probably much lower and can thus, together with the resulting shallower clay layer, explain that Elymus is limited by nitrogen in these salt marshes.…”

“…Sediment deposition rate in our study area is 2.8–8.3 kg m −2 year −1 (Nolte, Müller et al., ). Furthermore, we know that the N content of the freshly deposited sediment collected using sediment traps is around 0.28% of the total mass (Mueller, Do, Jensen, & Nolte, ). Therefore the input of N via sediment deposition in the study site ranges roughly between 7.8 and 23.0 g m −2 year −1 and thus is representative of the lower N fertilization treatment.…”

Movement of Microphytobenthos and Sediment Between Mudflats and Salt Marsh During Spring Tides

“…Salt marshes provide a range of ecosystem functions, such as denitrification and nutrient biogeochemistry (Seitzinger, 1973;Ogilvie et al, 1997;Underwood, 1997;Cibic et al, 2007), primary production and carbon storage (Macreadie et al, 2013;Mueller et al, 2019), wave attenuation and coastal protection (Shepard et al, 2011;Yang et al, 2012), food provisioning (Grothues and Able, 2003;Banerjee et al, 2017), and cultural benefits that are important ecosystem services for human society. There has been significant research into how to protect, enhance, and restore salt marshes to preserve these functions in the face of changing environmental pressures on coasts (Reed et al, 2018).…”