Dynamics of extractable organic carbon in spodosol forest floors

Transport of dissolved organic carbon (DOC) from the forest floor to the mineral soil in Spodosols affects many biogeochemical processes. The nature of the C reservoir in organic soil immediately before leaching, however, is not clear. Extractable organic C may partition between dissolved and undissolved phases, or reside in entrained water prior to leaching. We used laboratory incubations and extractions to test which model more accurately describes the reservoir from which DOC is removed. We also measured the rate at which C enters the extractable reservoir, how that rate changes with time, and the partitioning relationship between this reservoir and bulk solution. In addition, we tested whether the composition of extractable DOC changes with time. Extractable organic C was stored in an adsorbed phase. DOC in a solution of hydrophobic acids isolated from soil solution reduced dissolution of additional 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 consistent with storage of DOC in entrained water alone. Soil produced 48 micrograms g-1 h-1 of DOC when extracted repeatedly at 1-h intervals, but only 1 microgram g-1 h-1 when extracted at 1-week intervals. Extractable organic C production is fast (230 micrograms g-1 d-1) within 1 day of the last extraction, and then declines to a constant rate (15 micrograms g-1 d-1). Hydrophobic acids accounted for 75% of the extracted DOC after a 1-week incubation, but contributed only 50% of the increase in the extractable pool in subsequent weeks. Laboratory incubations and extractions of Oa material provided useful, consistent data for studying the dynamics of DOC removal from soil.