Low dissolved organic carbon input from fresh litter to deep mineral soils

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 C-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 C 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 C 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.