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Title: Vadose zone flow and transport of dissolved organic carbon at multiple scales in humid regimes

Author
item JARDINE, P. M. - OAK RIDGE NAT'L LAB.
item MAYES, M. A. - OAK RIDGE NAT'L LAB.
item MULHOLLAND, P. J. - OAK RIDGE NAT'L LAB.
item HANSON, P. J. - OAK RIDGE NAT'L LAB.
item TARVER, J. R. - OAK RIDGE NAT'L LAB.
item LUXMOORE, R. J. - OAK RIDGE NAT'L LAB.
item MCCARTHY, J. F. - UNIV. OF TENNESSEE
item Wilson, Glenn

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2006
Publication Date: 3/8/2006
Citation: Jardine, P., Mayes, M., Mulholland, P., Hanson, P., Tarver, J., Luxmoore, R., Mccarthy, J., Wilson, G.V. 2006. Vadose zone flow and transport of dissolved organic carbon at multiple scales in humid regions. Vadose Zone Journal, 5(1): 140-152.

Interpretive Summary: There is great concern over the impact of rising carbon dioxide levels in the earth's atmosphere because of human activity. These impacts can be alleviated by storage of carbon in the earths surface. Efforts to enhance the storage of organic carbon have traditionally focused on above-ground plant tissue and surface soils, however an unexplored potential exists to store organic carbon in deep subsoils. This paper presents a case study of the fate and movement of dissolved organic carbon (DOC) in a highly weathered soil based upon laboratory and field scale studies. The objective was to interpret the processes that control DOC movement observed at the different size studies to determine the potential for DOC storage in deep subsoils within a humid region. The approach involved laboratory-scale soil samples and undisturbed soil columns (0.2m x 1.0m), an intermediate-scale field soil profile (2m x 2m x 3m), a well-instrumented forested catchment (0.47 ha), and the stream system of the larger watershed (38.4 ha). Laboratory-scale experiments confirmed that the lower horizons have a huge capacity to store DOC but that rapid water flow through large pores tends to limit Carbon retention. Intermediate-scale experiments demonstrated the beneficial effects of movement through small pores within the soil to the storage of Carbon. Field- and watershed-scale studies demonstrated the complex interactions among processes that limit DOC storage in the subsoil and how these processes are dependent upon local environmental conditions. These results indicate the necessity for conducting experiments at a variety of scales to properly assess the capacity of soils to store organic Carbon. By knowing the basic organic Carbon storage processes, the potential to store organic Carbon in these soil systems can be predicted.

Technical Abstract: The necessity to offset global CO2 emissions is a reality and one that scientists must embrace regardless of politics. Efforts to enhance terrestrial organic carbon sequestration have traditionally focused on above-ground biomass and surface soils, however an unexplored potential exists in the thick lower horizons of widespread, highly developed mature soils such as Alfisols, Ultisols, and Oxisols. In the following manuscript we present a case study of the fate and transport of dissolved organic carbon (DOC) in a highly weathered Ultisol, involving spatial scales from the laboratory to the landscape. Our objectives are to interpret the processes observed at the various scales and provide an improved understanding of the coupled geochemical and hydrological mechanisms that control DOC mobility and sequestration in deep subsoils within high recharge, humid climatic regimes. Our approach involves a multiscale experimental endeavor using laboratory-scale batch and soil columns (0.2m x 1.0m), an intermediate-scale in situ pedon (2m x 2m x 3m), a well-instrumented subsurface field facility on a subwatershed (0.47 ha), and ephemeral and perennial stream discharge at the landscape scale (38.4 ha). Laboratory-scale experiments confirmed that the lower horizons have a huge propensity to accumulate and stabilize DOC on the solid phase, but that preferential fracture flow tends to limit C sequestration. Intermediate-scale experiments demonstrated the beneficial effects of matrix diffusion into micropores upon the sequestration of C. Field- and landscape-scale studies demonstrated the coupled hydrological, geochemical, and microbiological mechanisms that limit subsoil DOC sequestration, and the sensitivity of these mechanisms to local environmental conditions. Our results indicate the necessity of a multi-scale experimental approach to reasonably assess the propensity of deep subsoils to sequester organic C in situ. By unraveling fundamental organic C sequestration mechanisms in these soils, we improve the overall conceptual understanding and quantitative relationships needed to predict and alter organic C inventories and budgets in soil systems.