Soil Carbon Stocks Vary Across Geomorphic Settings in Australian Temperate Tidal Marsh Ecosystems

Tidal marshes rank among the ecosystems with the highest capacity to sequester and store organic carbon (C ) on earth. To inform conservation of coastal vegetated ecosystems for climate change mitigation, this study investigated the factors driving variability in carbon storage. We estimated soil C stocks in tidal marshes across temperate Western Australia and assessed differences among geomorphic settings (marine and fluvial deltas, and mid-estuary) and vegetation type (Sarcocornia quinqueflora and Juncus kraussii) linked to soil biogeochemistry. Soil C stocks within fluvial and mid-estuary settings were significantly higher (209 ± 14 and 211 ± 20 Mg C ha , respectively; 1-m-thick soils) than in marine counterparts (156 ± 12 Mg C ha ), which can be partially explained by higher preservation of soil C in fluvial and mid-estuary settings rich in fine-grained (< 0.063 mm) sediments (49 ± 3% and 47 ± 4%, respectively) compared to marine settings (23 ± 4%). Soil C stocks were not significantly different between S. quinqueflora and J. kraussii marshes (185 ± 13 and 202 ± 13 Mg C ha , respectively). The higher contribution of tidal marsh plus supratidal vegetation in fluvial (80%) and intermediate (76%) compared to marine (57%) settings further explains differences in soil C stocks. The estimated soil C stocks in temperate Western Australia’s tidal marshes (57 Tg C within ~ 3000 km extent) correspond to about 2% of worldwide tidal marsh soil C stocks. The results obtained identify global drivers of soil C storage in tidal marshes and can be used to target hot spots for climate change mitigation based on tidal marsh conservation. org org org org org org org org org org org org org −1 −1 −1 2