Fine-root biomass and fluxes of soil carbon in young stands of paper birchand trembling aspen as affected by elevated atmospheric Co-2 and tropospheric O-3

Rising atmospheric CO2 may stimulate future forest productivity, possibly i ncreasing carbon storage in terrestrial ecosystems, but how tropospheric oz one will modify this response is unknown. Because of the importance of fine roots to the belowground C cycle, we monitored fine-root biomass and assoc iated C fluxes in regenerating stands of trembling aspen, and mixed stands of trembling aspen and paper birch at FACTS-II, the Aspen FACE project in R hinelander, Wisconsin. Free-air CO2 enrichment (FACE) was used to elevate c oncentrations of CO2 (average enrichment concentration 535 mul l(-1)) and O -3 (53 nl l(-1)) in developing forest stands in 1998 and 1999. Soil respira tion, soil pCO(2), and dissolved organic carbon in soil solution (DOC) were monitored biweekly. Soil respiration was measured with a portable infrared gas analyzer. Soil pCO(2) and DOC samples were collected from soil gas wel ls and tension lysimeters, respectively. at depths of 15, 30, and 125 cm. F ine-root biomass averaged 263 g m(-2) in control plots and increased 96% un der elevated CO2. The increased root biomass was accompanied by a 39% incre ase in soil respiration and a 27% increase in soil pCO(2). Both soil respir ation and pCO(2) exhibited a strong seasonal signal, which was positively c orrelated with soil temperature. DOC concentrations in soil solution averag ed similar to 12 mg l(-1) in surface horizons, declined with depth, and wer e little affected by the treatments. A simplified belowground C budget for the site indicated that native soil organic matter still dominated the syst em, and that soil respiration was by far the largest flux. Ozone decreased the above responses to elevated CO2, but effects were rarely statistically significant. We conclude that regenerating stands of northern hardwoods hav e the potential for substantially greater C input to soil due to greater fi ne-root production under elevated CO2 Greater fine-root biomass will be acc ompanied by greater soil C efflux as soil respiration, but leaching losses of C will probably be unaffected.