DYNAMICS OF EXTRACTABLE ORGANIC-CARBON IN SPODOSOL FOREST FLOORS

Transport of dissolved organic carbon (DOG) from the forest floor to t he mineral soil in Spodosols affects many biogeochemical processes. Th e nature of the C reservoir in organic soil immediately before leachin g, however, is not clear. Extractable organic C may partition between dissolved and undissolved phases, or reside in entrained water prior t o leaching. We used laboratory incubations and extractions to test whi ch model more accurately describes the reservoir from which DDC is rem oved. We also measured the rate at which C enters the extractable rese rvoir, how that rate changes with time, and the partitioning relations hip between this reservoir and bulk solution. In addition, we tested w hether the composition of extractable DOC changes with time. Extractab le organic C was stored in an adsorbed phase. DOC in a solution of hyd rophobic acids isolated from soil solution reduced dissolution of addi tional DOC by 41-44%. Furthermore, in each extraction, approximately 1 mg DOC remained in the unextracted phase for every 16-29 mg DOC l(-1) in the aqueous phase. This partitioning relationship was not consiste nt with storage of DOC in entrained water alone. Soil produced 48 mu g g(-1) h(-1) of DOC when extracted repeatedly at 1-h intervals, but on ly 1 mu g g(-1) h(-1) when extracted at 1-week intervals. Extractable organic C production is fast (230 mu g g(-1) d(-1)) within 1 day of th e last extraction, and then declines to a constant rate (15 mu g g(-1) d(-1)). Hydrophobic acids accounted for 75% of the extracted DOC afte r a 1-week incubation, but contributed only 50% of the increase in the extractable pool in subsequent weeks. Laboratory incubations and extr actions of Oa material provided useful, consistent data for studying t he dynamics of DOC removal from soil. Copyright (C) 1996 Elsevier Scie nce Ltd