LAUSR

Tidal marshes rank among the ecosystems with the highest capacity to sequester and store organic carbon (C org ) 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 org 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 org stocks within fluvial and mid-estuary settings were significantly higher (209 ± 14 and 211 ± 20 Mg C org  ha −1 , respectively; 1-m-thick soils) than in marine counterparts (156 ± 12 Mg C org  ha −1 ), which can be partially explained by higher preservation of soil C org 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 org stocks were not significantly different between S. quinqueflora and J. kraussii marshes (185 ± 13 and 202 ± 13 Mg C org  ha −1 , 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 org stocks. The estimated soil C org stocks in temperate Western Australia’s tidal marshes (57 Tg C org within ~ 3000 km 2 extent) correspond to about 2% of worldwide tidal marsh soil C org stocks. The results obtained identify global drivers of soil C org storage in tidal marshes and can be used to target hot spots for climate change mitigation based on tidal marsh conservation.