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Abstract

Found in the coastal intertidal transition zone between the land and ocean throughout global temperate coastlines, tidal marshes absorb and assimilate carbon dioxide at a rate equal to or higher than the most productive ecosystems on Earth. The inherent ecological processes that create and sustain tidal marshes are also responsible for its carbon storage ability. In addition to high primary production, marsh vegetation traps suspended sediments brought in by ocean tides and terrestrial rivers, accreting organic matter vertically in a highly dynamic process. Despite growing mapping and sampling efforts globally, it is unclear how much the carbon storage capacity of tidal marshes varies with environmental influences, and marshes on the coast of British Columbia (BC) have received less attention in existing studies and data. Here, I collate and synthesize belowground measurements of soil organic carbon (SOC) stocks (Mg C per ha) from soil cores collected across BC tidal marshes. I estimate that BC’s tidal marshes contain 4.7-7.3 Tg of C in shallow SOC stocks (30 cm), and 11.1-17.2 Tg of C in deep SOC stocks (100 cm). I find that SOC stocks in BC’s tidal marshes are consistent with other North American and Pacific coast marshes. My analysis quantifies variation in belowground measurements with environmental factors, applying novel metrics designed to describe imported carbon burial processes, in addition to conditions of marsh zonation, vegetation, ocean, and topography. I find that SOC stocks in BC’s tidal marshes were greater in the high marsh zone, increased with latitude, maximum NDVI (above-ground biomass), and distance from the low water line (LWL), while higher fetch (wind and wave exposure), slope, and distance from a major river outlet, generally produced lower SOC stocks. My study sheds light on the importance of analyzing ecologically informed covariates, including stability of the marsh and terrestrial inputs of organic carbon in tidal marshes. While improving the understanding of drivers of variation behind belowground marsh carbon storage, my study also reveals important data gaps in Pacific tidal marshes, particularly concerning soil depth and extent. This data-synthesis approach can inform strategies to enhance SOC storage through targeted management, restoration, and conservation.