Effects of CO(2) enrichment on growth and root (15)NH(4) (+) uptake rate of loblolly pine and ponderosa pine seedlings.

We examined changes in root growth and (15)NH(4) (+) uptake capacity of loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Douglas. Ex Laws.) seedlings that were grown in pots in a phytotron at CO(2) partial pressures of 35 or 70 Pa with NH(4) (+) as the sole N source. Kinetics of (15)N-labeled NH(4) (+) uptake were determined in excised roots, whereas total NH(4) (+) uptake and uptake rates were determined in intact root systems following a 48-h labeling of intact seedlings with (15)N. In both species, the elevated CO(2) treatment caused a significant downregulation of (15)NH(4) (+) uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root (15)NH(4) (+) uptake (V(max)). Rates of (15)NH(4) (+) uptake in intact roots were, however, unaffected by CO(2) treatment and were on average 4- to 10-fold less than the V(max) in excised roots, suggesting that (15)NH(4) (+) absorption from the soil was not limited by the kinetics of root (15)NH(4) (+) uptake. Despite the lack of a CO(2) effect on intact root absorption rates, (15)NH(4) (+) uptake on a per plant basis was enhanced at high CO(2) concentrations in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO(2) concentration increased total (15)NH(4) (+) uptake and the fraction of total biomass allocated to fine roots (< 2 mm in diameter) to a similar relative extent. We suggest that the increased uptake on a per plant basis in response to CO(2) enrichment is largely the result of a compensatory increase in root absorbing surfaces.