Long-term Effects of Free Air CO2 Enrichment (FACE) on Soil Respiration

Emissions of CO₂ from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO₂ to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO₂ concentration affects soil CO₂ dynamics over 7 years of continuous enrichment. Soil respiration, soil CO₂ concentrations, and the isotopic signature of soil CO₂ were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO₂ efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO₂ enrichment effect on soil CO₂ efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO₂ concentrations and depletion in the δ¹³C of soil CO₂ at all but the shallowest soil depths.