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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Impact of Sedimentation on Wetland Carbon Sequestration in an Agricultural Watershed

^a USDA-ARS Hydrology & Remote Sensing Lab. Building 007 BARC-West, Beltsville, MD 20705
^b USDA-ARS Environmental Microbial Safety Laboratory Building 173 BARC-East Beltsville, MD 20705. Trade names are included for the benefit of the reader and do not imply an endorsement of or a preference for the product listed by the U.S. Dep. of Agriculture

^* Corresponding author (greg.mccarty{at}ars.usda.gov).

Received for publication January 9, 2008. Landscape redistribution of soil C is common within agricultural ecosystems. Little is known about the effects of upland sediment deposition on C dynamics within riparian wetlands. To assess sedimentation impact, we obtained profile samples of wetland soil and used the combination of ¹³⁷Cs, ²¹⁰Pb, and ¹⁴C chronological markers to determine rates of C sequestration and mineral deposition over the history of a wetland within a first-order catchment under agricultural management in the coastal plains of the United States. Substantial post settlement deposition in the wetland soil was evidenced in places by a 20- to 40-cm layer of mineral soil that buried the original histosol. Soil profiles contained a minimum in C content within the top 35 cm of the profile which originated from a rapid deposition from low C upland soils. Radiocarbon and radioisotope dating showed that increases in C above this minimum were the result of C sequestered in the past 50 yr. Modeling the kinetics of modern C dynamics using the ¹³⁷Cs and ²¹⁰Pb markers within these surface profiles provides strong evidence for accelerated C sequestration associated with mineral sediment deposition in the ecosystem. These findings indicate that at the landscape scale, dilution of ecosystem C by import of low C upland sediment into wetlands stimulates C sequestration by pulling soil C content below some pedogenic equilibrium value for the ecosystem. They also indicate that over the history of the wetland, rates of C accretion may be linked to mineral soil deposition.