ABOVEGROUND AND BELOWGROUND RESPONSE OF POPULUS GRANDIDENTATA TO ELEVATED ATMOSPHERIC CO2 AND SOIL N-AVAILABILITY

Soil N availability may play an important role in regulating the long- term responses of plants to rising atmospheric CO2 partial pressure. T o further examine the linkage between above- and belowground C and N c ycles at elevated CO2, we grew clonally propagated cuttings of Populus grandidentata in the field at ambient and twice ambient CO2 in open b ottom root boxes filled with organic matter poor native soil. Nitrogen was added to all root boxes at a rate equivalent to net N mineralizat ion in local dry oak forests. Nitrogen added during August was enriche d with N-25 to trace the flux of N within the plant-soil system. Above - and belowground growth, CO2 assimilation, and leaf N content were me asured non-destructively over 142 d. After final destructive harvest, roots, stems, and leaves were analyzed for total N and N-15. There was no CO2 treatment effect on leaf area, root length, or net assimilatio n prior to the completion of N addition. Following the N addition, lea f N content increased in both CO2 treatments, but net assimilation sho wed a sustained increase only in elevated CO2 grown plants. Root relat ive extension rate was greater at elevated CO2, both before and after the N addition. Although final root biomass was greater at elevated CO 2, there was no CO2 effect on plant N uptake or allocation. While low soil N availability severely inhibited CO2 responses, high CO2 grown p lants were more responsive to N. This differential behavior must be co nsidered in light of the temporal and spatial heterogeneity of soil re sources, particularly N which often limits plant growth in temperate f orests.