The capacity of roadside vegetated filter strips and swales to sequester carbon

► Carbon density of North Carolina highway stormwater control measures were studied. ► Piedmont VFS/VSs sequestered C at a rate of 0.09kgC/m2/yr (segmented model). ► Wetland swales were found to have higher carbon density than dry swales. Carbon capture and storage within vegetation and soil is impacted by changing land uses, which results in either a net source or sink of greenhouse gases (GHGs) to the atmosphere. Transportation corridors are present world-wide, and the vegetated filter strip and vegetated swale (VFS/VS), a common stormwater control measure, often constitutes the right-of-way (ROW) adjacent to roadways. The roadway environment, specifically carbon pools in North Carolina highway ROWs, were studied for carbon sequestration potential, an important ecosystem service. The study was conducted in two North Carolina physiographic regions: the Piedmont (characterized by clay-influenced soils) and the Coastal Plain (predominantly sandy soils). Approximately 700 soil samples were collected in VFS/VSs and wetland swales alongside major highways and analyzed for percent total soil C (% total C) and bulk density to obtain the C density. Mean soil C densities (per unit area) were 2.55±0.13kgCm−2 (mean±standard error, n=160, 0.2m sample depth) in the Piedmont and 4.14±0.15kgCm−2 (n=160, 0.2m depth) in Coastal Plain highway VFS/VSs. Previous studies on grasslands had similar C density values to those observed in this study; thus, grasslands could be a surrogate land use for highway VFS/VSs. A thirty-seven year soil chronosequence characterized C accumulation in Piedmont VFS/VSs. Carbon density increases showed an association with age in Piedmont VFS/VSs only, which were calculated to reach maximum C density of 3.34kgCm−2, at age=21.5 years. Previous studies on grasslands show similar C density and accumulation values to those observed in this study; thus, again grasslands could be a surrogate land use for highway VFS/VSs. Carbon density did not differ between dry or wetland swales, although % total C was significantly greater in wetland swales. The mean VS C density was 3.05±0.13kgCm−2 (n=40, 0.2m depth), while that for wetland swales was 5.04±0.73kgCm−2 (n=44, 0.2m depth). To promote C sequestration in the vegetated ROW, wetland swales appear preferable to dry swales.