PNNL

Abstract

Despite growing interest in the contributions of tidal wetlands to natural climate solutions, data remain scarce on how land use affects their carbon accumulation rates (CAR). Additionally, environmental factors driving the large observed variability in CAR among sites are poorly understood. To address these knowledge gaps, we measured CAR using 210Pb profiles and surface elevation tables from 39 wetland sites in seven estuaries in Oregon and Washington, USA. Each estuary contained one or more sites varying in wetland type (tidal marsh, tidal swamp, nontidal pasture), land-use history (reference, restored, and disturbed), and salinity (fresh to polyhaline). Carbon density was highest in forested tidal swamps, while CAR and accretion rates were highest in restored marshes and lowest in nontidal pastures. Variation in CAR correlated strongly with sediment accretion rates but not with soil carbon density. Site-level environmental variables considered as potential predictors of CAR included water table level, water temperature, salinity, soil temperature, tidal elevation, and distance to channel. Estuary-level variables included latitude, relative sea-level rise rate, riverine sediment load relative to estuary area, and watershed size. Growing season water table level and latitude were the strongest drivers, together explaining 18% of CAR variance. When only tidal sites were considered, water table level alone explained 19% of variance, with higher CAR observed when mean growing season water table levels were near or above the soil surface (> -0.061 m). These findings provide new insight into the biophysical drivers of accretion and CAR across a diverse set of tidal wetlands, illustrating the importance of hydrologic processes. Our results also highlight the potential of restored tidal wetland sites to rapidly accumulate new soil carbon through high rates of accretion.