Endogenous and Exogenous Carbon Sequestration in a Constructed Flow-through Wetland Receiving Agricultural Runoff.

In the U.S., an estimated one third of all eroded material is deposited in small depositional environments. Constructed wetlands are being used in California to treat agricultural runoff, and may serve as important carbon sinks. This study examined the spatial and temporal dynamics of carbon and sediment accumulation to evaluate the potential for carbon sequestration in a thirteen-year-old seasonally saturated constructed wetland. Net sedimentation and net above/below-ground biomass were measured in 2004 and 2005. Sediment cores were collected to the antecedent (time zero) soil layer, which ranged between 2 and 50 cm below the soil surface.  All samples were analyzed for total carbon. The spatial variability of carbon and sediment accumulation was modeled with geostatistics. Average sediment accumulation rate, nearly quadrupled from 2004 to 2005, with rates of 2.7 kg m^-2y^-1 (range: 0-11 kg m^-2y^-1) in 2004 and 10.0 kg m^-2y^-1 (range: 0-97 kg m^-2y^-1) in 2005. Average carbon accumulation rate did not change between years, with rates of 0.210 kg m^-2 y^-1 in 2004 and 0.249 kg m^-2y^-1 in 2005, indicating a change in carbon source between years. Average carbon concentration in all accumulated sediment was 14 g kg^-1, which ranged from 10 g kg^-1 in sediment directly overlaying the antecedent layer to 24 g kg^-1 in the top 2.5 cm of the wetland surface. Average carbon content of inflowing sediment was 14 g kg^-1, indicating an enrichment of surface sediment with endogenous carbon sources. Due to fluctuating cycles of flooding and drying, however, the long-term storage of carbon (~14 g kg^-1) is maintained at the same level as inflowing sediment (14 g kg^-1), indicating no additional long-term storage. This study shows that constructed flow-through wetlands can play an important role in the storage of eroded carbon, however, additional sequestration from endogenous sources may be limited in seasonally-saturated wetlands due to enhanced oxidation during drying cycles.