Effects of CO2 enrichment on growth and root 15NH4+ uptake rat

NH4 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 CO2 partial pressures of 35 or 70 Pa with NH4 + as the sole N source. Kinetics of N-labeled NH4 + uptake were determined in excised roots, whereas total NH4 + uptake and uptake rates were determined in intact root systems following a 48-h labeling of intact seedlings with N. In both species, the elevated CO2 treatment caused a significant downregulation of NH4 + uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root NH4 + uptake (Vmax). Rates of NH4 + uptake in intact roots were, however, unaffected by CO2 treatment and were on average 4to 10-fold less than the Vmax in excised roots, suggesting that NH4 + absorption from the soil was not limited by the kinetics of root NH4 + uptake. Despite the lack of a CO2 effect on intact root absorption rates, NH4 + uptake on a per plant basis was enhanced at high CO2 concentrations in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO2 concentration increased total NH4 + 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 CO2 enrichment is largely the result of a compensatory increase in root absorbing surfaces.