Effects of free-air CO2 enrichment (FACE) on belowground processes in a Pinus taeda forest

Terrestrial vegetation and soils may act as important carbon sinks if rising atmospheric CO2 stimulates plant production. We used free-air CO2 enrichment (FACE) technology to expose three 30 m diameter plots of a loblolly pine (Pinus taeda) forest to elevated CO2 at 200 mu L/L above ambient levels, while three control plots were outfitted with FACE apparatus but were fumigated with ambient air. We quantified litterfall mass and chemistry, fine root biomass increment and turnover, CO2 efflux from soils, delta(13)C in soil CO2 soil CO2, soil microbial biomass C and N, and potential net N mineralization. After two growing seasons, elevated CO2 caused significant increases in loblolly pine litterfall mass and fine root increment. Within the first year of FACE treatment, the concentration of CO2 in soil had increased, and soil surface CO2 efflux was generally higher at elevated CO2, but this difference was not statistically significant. Loblolly pine litter C:N ratio, fine root turnover, microbial biomass C and N, and potential net N mineralization were not significantly affected by elevated CO2. Our results suggest that elevated atmospheric CO2 may accelerate inputs of organic matter to soil C pools in loblolly pine forests, but it may also accelerate losses of C from belowground by stimulating soil respiration.