Growth and carbon accumulation in root systems of Pinus taeda and Pinus ponderosa seedlings as affected by varying CO2, temperature and nitrogen

It has been hypothesized that increasing atmospheric CO2 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 CO2, 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 CO2 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 NH4NO3 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 CO2 response (105% increase) (P = 0.0001) as a result of significant increases in all root fractions in the elevated CO2 treatment, but all other main effects and interactions were insignificant. In I? taeda, there were significant interactions between CO2 and temperature (P = 0.04) and CO2 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 CO2. We conclude that both species have the potential to increase be lowground biomass substantially in response to rising atmospheric CO2 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.