Abstract
Most experimental studies measuring the effects of climate change on terrestrial C cycling have focused on processes that occur at relatively short time scales (up to a few years). However, climate-soil C interactions are influenced over much longer time scales by bioturbation and soil weathering affecting soil fertility, ecosystem productivity, and C storage. Elevated CO(2)can increase belowground C inputs and stimulate soil biota, potentially affecting bioturbation, and can decrease soil pH which could accelerate soil weathering rates. To determine whether we could resolve any changes in bioturbation or C storage, we investigated soil profiles collected from ambient and elevated-CO(2)plots at the Free-Air Carbon-Dioxide Enrichment (FACE) forest site at Oak Ridge National Laboratory after 11 years of (13)C-depleted CO(2) release. Profiles of organic carbon concentration, δ(13)C values, and activities of (137)Cs, (210)Pb, and (226)Ra were measured to ∼30 cm depth in replicated soil cores to evaluate the effects of elevated CO(2) on these parameters. Bioturbation models based on fitting advection-diffusion equations to (137)Cs and (210)Pb profiles showed that ambient and elevated-CO(2) plots had indistinguishable ranges of apparent biodiffusion constants, advection rates, and soil mixing times, although apparent biodiffusion constants and advection rates were larger for (137)Cs than for (210)Pb as is generally observed in soils. Temporal changes in profiles of δ(13)C values of soil organic carbon (SOC) suggest that addition of new SOC at depth was occurring at a faster rate than that implied by the net advection term of the bioturbation model. Ratios of ((210)Pb/(226)Ra) may indicate apparent soil mixing cells that are consistent with biological mechanisms, possibly earthworms and root proliferation, driving C addition and the mixing of soil between ∼4 cm and ∼18 cm depth. Burial of SOC by soil mixing processes could substantially increase the net long-term storage of soil C and should be incorporated in soil-atmosphere interaction models.