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 NH^sub 4^^sup +^ and NO^sub 3^^sup -^ were examined for seedlings of loblolly and ponderosa pine grown for 160 days under two CO^sub 2^ levels, ambient (35 Pa) and ambient plus 35 Pa (70 Pa). Fraction of biomass allocated to active fine roots as well as total N (NH^sub 4^^sup +^ + NO^sub 3^^sup -^ ) absorption per unit root dry mass were unaffected by CO^sub 2^. On a whole-plant basis, elevated CO^sub 2^ led to a significant increase in N acquisition in loblolly but not in ponderosa pine. However, even in loblolly pine where CO^sub 2^ 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 CO^sub 2^. We suggest that the commonly reported decline in tissue N concentration of these and other species at high CO^sub 2^ is largely caused by inability of the root systems to sufficiently compensate for increased N demand. Elevated CO^sub 2^ significantly altered root uptake capacity of the different N forms, i.e., high CO^sub 2^ significantly increased NO^sub 3^^sup -^ absorption rates, but decreased NH^sub 4^^sup +^ absorption rates in both species though the decrease in loblolly was insignificant. However, elevated CO^sub 2^ increased root respiration rate in loblolly pine while significantly decreasing it in ponderosa pine. This indicates that CO^sub 2^-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 CO^sub 2^, the results could have important implications for N dynamics in managed and natural plant communities.[PUBLICATION ABSTRACT]