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Effects of Elevated CO2 and O3 on Belowground Carbon Cycling in Aspen and Birch Stands
Reference
King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands, J.G., Dickson, R.E., Hendrey, G.R. and Karnosky, D.F.  2001.  Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3Oecologia 128: 237-250.

What was done
The authors grew O3-sensitive and O3-tolerant aspen (Populus tremuloides Michx.) clones alone and in mixed stands of paper birch (Betula papyrifera Marsh.) for two years in 30-m diameter FACE plots located in Wisconsin, USA, at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times ambient ozone concentration) to study the interactive effects of these parameters on fine root production and belowground carbon cycling in soils associated with these stands.

What was learned
Elevated CO2 significantly enhanced the production of fine root biomass by 133 and 83% for aspen and aspen-birch mixed stands, respectively.  In contrast, elevated O3 had no effect on fine-root biomass, while simultaneous exposure to elevated CO2 and O3 increased fine-root biomass by approximately 66% for both types of stands.  Averaged across both stands, elevated CO2 also increased dead root biomass by 140%, which is but another example of how elevated CO2 increases carbon inputs to soils.  Together, these two phenomena contributed to a CO2-mediated increase in the proportion of total soil carbon comprised of fine roots and their associated mycorrhizae (live and dead) in the forest soils, increasing this parameter from 8% at ambient CO2 to 15% at elevated CO2.

Elevated CO2 also increased atmospheric CO2 concentrations within the soil profile by an average of 27% down to 125 cm.  In addition, soil respiration, which represented the largest efflux of carbon from the forest soils, was increased by an average of 39%, due to enhanced biological activity resulting from increased soil carbon inputs.  However, say the authors, such an increase in soil respiration "does not preclude greater long-term C storage in forest soils of the future," but must be weighed against carbon inputs from root turnover, exudation, and leaf senescence, which appear to be significantly larger in magnitude than respiratory carbon losses.

What it means
As the tropospheric ozone concentration continues to increase, it will likely negatively affect plant photosynthetic rates and other physiological processes.  Fortunately, the rising CO2 content of the atmosphere should provide plants, including trembling aspen and birch seedlings, with an increasing degree of protection from this harmful aerial pollutant.  Indeed, although CO2-induced increases in fine root biomass were reduced by elevated O3 concentrations, the remaining level of CO2-induced stimulation in root growth (66%) demonstrated that the positive effects of elevated CO2 were far from eliminated.

Increases in the air's CO2 concentration will also likely increase rates of soil respiration.  However, rates of soil carbon loss via this mechanism will likely be much lower than rates of soil carbon gains under conditions of elevated CO2.  Hence, it is likely that forest soils beneath regenerating stands of aspen and paper birch will exhibit a greater propensity to sequester carbon as atmospheric CO2 concentrations continue to rise.