Dissolved organic carbon (DOC) leached from recent litter in the forest floor has been suggested to be an important source of C to the mineral soil of forest ecosystems. To determine the rate at which this flux of C occurs, we have taken advantage of a local release of 14C at Oak Ridge National Laboratory Reservation, Oak Ridge, TN (35°58′N, 84°16′W). Eight replicate 7‐ by 7‐m plots were established at four field sites on the reservation in an upland oak forest setting. Half of the plots were provided with 14C‐enriched litter (Δ14C ≈ 1000‰), and the other half with near‐background litter (Δ14C ≈ 220‰) for multiple years. Differences in the labeled leaf litter were used to quantify the movement of litter‐derived DOC through the soil profile. Soil solutions were collected for several years with tension lysimeters at 15‐ and 70‐cm depths and measured for DOC concentration and 14C abundance. The net amount of DOC retained between 15 and 70 cm was between 2 and 10 g m−2 yr−1 There were significant effects of the litter additions on the 14C abundance in the DOC, but the net transport of 14C from the added litter was small. The difference in Δ14C between the treatments with enriched and near‐background litter was only about 130‰ at both depths, which is small compared with the difference in Δ14C in the added litter. The primary source of DOC within the mineral soil must therefore have been either the Oe or Oa horizon or the organic matter in the mineral soil. During a 2‐yr time frame, leaching of DOC from recent litter did not have a major impact on the C stock in the mineral soil below 15 cm in this ecosystem.
Scientists must embrace the necessity to offset global CO 2 emissions regardless of politics. Efforts to enhance terrestrial organic carbon sequestration have traditionally focused on aboveground biomass and surface soils. An unexplored potential exists in thick lower horizons of widespread, mature soils such as Alfisols, Ultisols, and Oxisols. We present a case study of fate and transport of dissolved organic carbon (DOC) in a highly weathered Ultisol, involving spatial scales from the laboratory to the landscape. Our objectives were to interpret processes observed at various scales and provide an improved understanding of coupled hydrogeochemical mechanisms that control DOC mobility and sequestration in deep subsoils within humid climatic regimes. Our approach is multiscale, using laboratory-scale batch and soil columns (0.2 by 1.0 m), an in situ pedon (2 by 2 by 3 m), a well-instrumented subsurface facility on a subwatershed (0.47 ha), and ephemeral and perennial stream discharge at the landscape scale (38.4 ha). Laboratoryscale experiments confirmed that lower horizons have the propensity to accumulate DOC, but that preferential fracture flow tends to limit sequestration. Intermediate-scale experiments demonstrated the beneficial effects of C diffusion into soil micropores. Field-and landscapescale studies demonstrated coupled hydrological, geochemical, and microbiological mechanisms that limit DOC sequestration, and their sensitivity to local environmental conditions. Our results suggest a multi-scale approach is necessary to assess the propensity of deep subsoils to sequester organic C in situ. By unraveling fundamental organic C sequestration mechanisms, we improve the conceptual and quantitative understanding needed to predict and alter organic C budgets in soil systems.
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