Growth and carbon accumulation in root systems of Pinus taeda and Pinus ponderosa seedlings as affected by varying CO(2), temperature and nitrogen.

It has been hypothesized that increasing atmospheric CO(2) concentration enhances accumulation of carbon in fine roots, thereby altering soil carbon dynamics and nutrient cycling. To evaluate possible changes to belowground pools of carbon and nitrogen in response to elevated CO(2), an early and a late successional species of pine (Pinus taeda L. and Pinus ponderosa Dougl. ex Laws, respectively) were grown from seed for 160 days in a 35 or 70 Pa CO(2) partial pressure at low or high temperature (30-year weekly mean and 30-year weekly mean + 5 degrees C) and a soil solution nitrogen concentration of 1 or 5 mM NH(4)NO(3) at the Duke University Phytotron. Seedlings were harvested at monthly intervals and growth parameters of the primary root, secondary root and tap root fractions evaluated. Total root biomass of P. ponderosa showed a positive CO(2) response (105% increase) (P = 0.0001) as a result of significant increases in all root fractions in the elevated CO(2) treatment, but all other main effects and interactions were insignificant. In P. taeda, there were significant interactions between CO(2) and temperature (P = 0.04) and CO(2) and nitrogen (P = 0.04) for total root biomass. An allometric analysis indicated that modulation of the secondary root fraction was the main response of the trees to altered environmental conditions. In P. ponderosa, there was an increase in the secondary root fraction relative to the primary and tap root fractions under conditions of low temperature. In P. taeda, there was a shift in carbon accumulation to the secondary roots relative to the primary roots under low temperature and low nitrogen. Neither species exhibited shifts in carbon accumulation in response to elevated CO(2). We conclude that both species have the potential to increase belowground biomass substantially in response to rising atmospheric CO(2) concentration, and this response is sensitive to temperature and nitrogen in P. taeda. Both species displayed small shifts in belowground carbon accumulation in response to altered temperature and nitrogen that may have substantial ecosystem consequences over time.