Changes in root NH ⁺₄ and NO ⁻₃ absorption rates of loblolly and ponderosa pine in response to CO₂ enrichment

Abstract

Root growth and physiological uptake capacity for NH ⁺₄ and NO ⁻₃ 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 (NH ⁺₄ + NO ⁻₃ ) 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 NO ⁻₃ absorption rates, but decreased NH ⁺₄ 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.