ARS | Publication request: Impact of Sedimentation on Wetland Carbon Sequestration in Agricultural Watershed

Submitted to: Global Biogeochemical Cycles
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 23, 2008
Publication Date: N/A

Interpretive Summary: There is debate in the literature concerning the impact of soil erosion on carbon dynamics within the terrestrial ecosystem. Soil erosion has a well established negative impact on soil quality and productivity of agricultural lands but its impact on balance of carbon stocks within terrestrial ecosystems is unclear. It has been estimated that 75 billion Mg y-1 of soil is subject to water erosion which results in displacement of perhaps 40 to 60 Pg y-1 of soil C. The influence of this massive redistribution of soil C on terrestrial C storage is poorly understood. Arguments for negative impacts of soil erosion on terrestrial carbon stocks center on the effects of erosion on ability of degraded soils to support plant growth and increased mineralization in displaced soil because of breakdown of aggregates leading to exposure of physically protected organic matter to degradation. Some have estimated that soil carbon mineralization has increased by greater than 20% as a result of erosion. In contrast, others put forward a conceptual framework for linking redistribution of soil resources to impacts on carbon cycle processes that result in increased carbon sequestration within the terrestrial ecosystem. Within this framework C is buried at sites of deposition resulting in stabilization of eroded carbon and removal of carbon at the site of erosion stimulates sequestration due to decreasing the carbon content below that an equilibrium carbon content. The reported work compares pre-modern rates of sedimentation and carbon sequestration of the wetland to modern rates primarily associated with agricultural land use within the small first order watershed under study. For this assessment we used a set of three nested chronological markers (137Cs, 210exPb and 14C) to assess rates of sediment deposition and carbon sequestration within the wetland ecosystem. The soil cores contained a local soil carbon minimum within the top 35 cm of the profile which is characteristic of a recent period of rapid deposition from upland soils. Radiocarbon dating showed that the soil C above the minimum was recently sequestered (modern C). The magnitude of the modern C pools within these profiles provides strong evidence for accelerated C sequestration associated with rapid mineral deposition.

Technical Abstract: Landscape redistribution of soil carbon is common within agricultural ecosystems. Associated riparian wetlands are often impacted by sediments from upland soils. Little is known about the effects of upland sediment deposition on carbon dynamics within riparian wetlands. To assess sedimentation impact, we obtained intact profile samples of wetland soil and used the combination of 137Cs, 210exPb, and 14C chronological markers to determine rates of carbon and mineral deposition over the history of a wetland within a first-order catchment under agricultural management. We characterized the wetland soil as having substantial post settlement deposition as evidenced in places by a substantial layer of mineral soil that buried the original histosol. More generally, the intact soil cores contained a local soil carbon minimum within the top 35 cm of the profile which is characteristic of a recent period of rapid deposition from upland soils. Radiocarbon dating showed that the soil C above the minimum was recently sequestered (modern C). The magnitude of the modern C pools within these profiles provides strong evidence for accelerated C sequestration associated with rapid mineral deposition.