Non-Floodplain Wetlands Are Carbon-Storage Powerhouses Across the United States

Understanding wetland carbon stores and dynamics are critical to managing global carbon flux. Non-floodplain wetlands (NFWs) are hydrologically dynamic and globally prevalent inland wetlands distal to fluvial flowpaths, lacustrine-fringing areas, and geomorphic floodplains; >50% the world's remaining wetlands have been reported as NFWs. Quantifying NFW carbon stores and dynamics represents a substantive global carbon-budget gap. We analyze conterminous-US (CONUS) field-based data collected from nearly 2000 wetland sites sampled by the National Wetlands Condition Assessment (NWCA) representing ∼38 Mha CONUS wetlands, asking: What is the mean soil organic carbon density and total carbon storage in different hydrogeomorphically classified wetland types? To what extent does soil organic carbon density and total carbon in NFWs differ from other wetland types? How does NFW soil organic carbon density and total carbon vary between altered and intact NFWs? We find that relative to other wetland types, NFWs are carbon-storing powerhouses, containing approximately 1.5x soil organic carbon per ha than other wetland types sampled. CONUS-wide, NFWs store more total carbon across every depth increment: ∼2.0x than other wetland types. Further, wetland condition affects carbon dynamics: least impaired NFWs had 1.6x the soil organic carbon density found in intermediately disturbed wetlands and 1.8x the density of most-disturbed NFWs. These NWCA data, plus waning societal protections, suggests that carbon releases from NFW destruction across CONUS landscapes are likely to increase—perhaps markedly—in the coming years (e.g., through altered hydrology affecting atmospheric release of NFW-stored carbon as well as dissolved carbon export).