TECHNICAL REPORTS 804 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 137 Cs, 210 Pb, and 14 C 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 137 Cs and 210 Pb markers within these surface proﬁles provides strong evidence for accelerated C sequestration associated with mineral sediment deposition in the ecosystem. Tese 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. Tey also indicate that over the history of the wetland, rates of C accretion may be linked to mineral soil deposition. Impact of Sedimentation on Wetland Carbon Sequestration in an Agricultural Watershed Gregory McCarty,* Yakov Pachepsky, and Jerry Ritchie USDA-ARS T here is debate in the literature concerning the impact of soil erosion on C dynamics within the terrestrial ecosystem. Soil erosion has a well established negative impact on soil quality and productivity of agricultural lands (Lal, 1998, 2002, 2003, 2004) but its impact on the balance of C stocks within terrestrial ecosystems is unclear. It has been estimated that 75 Pg yr –1 of soil is subject to water erosion (Lal, 2003) which results in displacement of perhaps 0.5 to 2 Pg yr –1 of soil C. Te inﬂuence of this massive redistribution of soil C on ter- restrial C storage is poorly understood. Arguments for negative impacts of soil erosion on terrestrial C stocks center on the eﬀects of erosion on the ability of degraded soils to support plant growth and increased mineralization in displaced soil because of break- down of aggregates leading to exposure of physically protected or- ganic matter to degradation agents (Lal, 1998, 2003). Some have estimated that soil C mineralization has increased by >20% as a result of erosion (Polyakov and Lal, 2004). In contrast, Stallard (1998) put forward a conceptual framework for linking redistri- bution of soil resources to changes in C cycle processes that result in increased C sequestration within terrestrial ecosystems. Within this framework pedogenic C is buried at sites of deposition result- ing in stabilization of eroded C and removal of C at the site of erosion stimulates sequestration due to decreasing the C content below that at pedogenic equilibrium C content. Subsequent ﬁeld measurements of C dynamics on highly erosive sites in Mississippi provided evidence for dynamic replacement of soil C indicating that a local C sink formed at the site of erosion (Harden et al., 1999; Van Oost et al., 2005). Watershed and basin scale studies are beginning to aggregate inﬂuence of patterns of soil redistribu- tion at scales larger than landscape (Smith et al., 2005). With the redistribution of C resources linked to erosion/depo- sition processes, there is considerable potential for systematic net movement of soil C resources along biogeochemical gradients as- sociated with topographic elements within the landscape (McCarty and Ritchie, 2002). For example, net movement of soil C into wetter areas of the landscape would tend to stabilize eroded C by decreas- ing the potential for C oxidation (Liu et al., 2003). In this context, much of the consideration for increased C storage is associated with passive mechanisms such as C burial which would also suppress C oxidation. Tis may be the main inﬂuence on C dynamics in ecosystems with relatively low productivity such as reservoirs. De- position in wetlands with high net primary productivity, however, G. McCarty, and J. Ritchie, USDA-ARS Hydrology & Remote Sensing Lab. Building 007 BARC-West, Beltsville, MD 20705; Y. Pachepsky, USDA-ARS Environmental Microbial Safety Laboratory Building 173 BARC-East Beltsville, MD 20705. Trade names are included for the beneft of the reader and do not imply an endorsement of or a preference for the product listed by the U.S. Dep. of Agriculture. Copyright © 2009 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including pho- tocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Published in J. Environ. Qual. 38:804–813 (2009). doi:10.2134/jeq2008.0012 Received Jan. 9 2008. *Corresponding author (greg.mccarty@ars.usda.gov). © ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA TECHNICAL REPORTS: WETLANDS AND AQUATIC PROCESSES