Does elevated atmospheric CO₂affect soil carbon burial and soil weathering in a forest ecosystem?

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 CO₂can 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-CO₂plots at the Free-Air Carbon-Dioxide Enrichment (FACE) forest site at Oak Ridge National Laboratory after 11 years of ¹³C-depleted CO₂ release. Profiles of organic carbon concentration, δ¹³C values, and activities of ¹³⁷Cs, ²¹⁰Pb, and ²²⁶Ra were measured to ∼30 cm depth in replicated soil cores to evaluate the effects of elevated CO₂ on these parameters. Bioturbation models based on fitting advection-diffusion equations to ¹³⁷Cs and ²¹⁰Pb profiles showed that ambient and elevated-CO₂ plots had indistinguishable ranges of apparent biodiffusion constants, advection rates, and soil mixing times, although apparent biodiffusion constants and advection rates were larger for ¹³⁷Cs than for ²¹⁰Pb as is generally observed in soils. Temporal changes in profiles of δ¹³C 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 (²¹⁰Pb/²²⁶Ra) 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.