Variability in sediment carbon stocks with tidal elevation, climate, and decomposition metrics in Pacific Northwest tidal marshes and forested wetlands

Coastal wetlands such as tidal marshes have a high capacity for long-term carbon sequestration, and can therefore contribute in part to climate change mitigation. Carbon sequestration and stocks are driven by local productivity, allochthonous inputs, and decomposition, which may vary spatially with climate, wetland type, and hydrologic regime. In a large regional data set from northern California to northern Washington, we compared sediment wetland carbon content (%), density, and stocks with wetland elevation, climate variables, and decomposition metrics to investigate potential drivers of long-term carbon storage in Pacific coast tidal wetlands. Among marshes, sites higher in the tidal frame had greater carbon content and density in surface sediments than lower elevation sites inundated more frequently, but carbon stocks in the top meter of sediments were only weakly correlated with elevation. In marshes and tidal forested wetlands, annual climate normals (precipitation and summer temperature) showed little relationship with carbon metrics, suggesting that other drivers are of greater importance to blue carbon variability at regional scales. Short-term decomposition rates varied among sites, but not consistently with near surface carbon content or density. Our findings suggest that wetland hydrology, which varies with intertidal elevation in Pacific coast tidal wetlands, could be a key driver of spatial variation in sediment carbon. Conservation and restoration of high tidal wetlands may lead to greater long-term coastal carbon storage than lower elevation sites.