Changes in root NH4+ and NO3- absorption rates of loblolly and ponderosa pine in response to CO2 enrichment

Root growth and physiological uptake capacity for NH4+ and NO3- were examined for seedlings of loblolly and ponderosa pine grown for 160 days under two CO2 levels, ambient (35 Pa) and ambient plus 35 Pa (70 Pa). Fraction of biomass allocated to active fine roots as well as total N (NH4+ + NO3-) absorption per unit root dry mass were unaffected by CO2. On a whole-plant basis, elevated CO2 led to a significant increase in N acquisition in loblolly but not in ponderosa pine. However, even in loblolly pine where CO2 significantly increased plant N acquisition, the relative increase, in biomass far exceeded the gain in N, i.e. a 60% increase in total dry weight was accompanied by only a 30% increase in N gain in response to high CO2. We suggest that the commonly reported decline in tissue N concentration of these and other species at high CO2 is largely caused by inability of the root systems to sufficiently compensate for increased N demand. Elevated CO2 significantly altered root uptake capacity of the different N forms, i.e., high CO2 significantly increased NO3- absorption rates, but decreased NH4+ absorption rates in both species though the decrease in loblolly was insignificant. However, elevated CO2 increased root respiration rate in loblolly pine while significantly decreasing it in ponderosa pine. This indicates that CO2-induced changes in plant preference for inorganic N forms is not simply regulated by root energy status. If changes in plant preference for inorganic N forms represent typical responses to elevated CO2, the results could have important implications for N dynamics in managed and natural plant communities.