EFFECTS OF CO2 ENRICHMENT ON GROWTH AND ROOT (NH4-N-15 UPTAKE RATE OFLOBLOLLY-PINE AND PONDEROSA PINE-SEEDLINGS())

We examined changes in root growth and (NH4+)-N-15 uptake capacity of loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Dou glas. Ex Laws.) seedlings that were grown in pots in a phytotron at CO 2 partial pressures of 35 or 70 Pa with NH4+ as the sole N source. Kin etics of N-15-labeled NH4+ uptake were determined in excised roots, wh ereas total NH4+ uptake and uptake rates were determined in intact roo t systems following a 48-h labeling of intact seedlings with N-15. In both species, the elevated CO2 treatment caused a significant downregu lation of (NH4+)-N-15 uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root (NH4+)-N-15 uptake (V- max). Rates of (NH4+)-N-15 uptake in intact roots were, however, unaff ected by CO2 treatment and were on average 4- to 10-fold less than the V-max in excised roots, suggesting that (NH4+)-N-15 absorption from t he soil was not limited by the kinetics of root (NH4+)-N-15 uptake. De spite the lack of a CO2 effect on intact root absorption rates, (NH4+) -N-15 uptake on a per plant basis was enhanced at high CO2 concentrati ons in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO2 concentration increa sed total (NH4+)-N-15 uptake and the fraction of total biomass allocat ed 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 ro ot absorbing surfaces.