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_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 km² 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. [ABSTRACT FROM AUTHOR]