PARENT SESSION
Poster Session 32: Biogeochemistry
Thursday, August 11, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal
Nitrogen-use efficiency of a sweetgum forest in elevated CO2.
Norby, Richard*,1, Iversen, Colleen2, 1 Oak Ridge National Laboratory, Oak Ridge, TN2 University of Tennessee, Knoxville, TN
ABSTRACT- The tight linkage between C and N metabolism in plants and the widespread limitation on forest productivity imposed by N availability mandate that N dynamics be an integral part of analyses of forest response to rising atmospheric [CO2]. We used the construct of nitrogen-use efficiency (NUE) to analyze the C-N relations of a closed-canopy Liquidambar styraciflua (sweetgum) plantation exposed to elevated (550 ppm) [CO2] for 6 years in a free-air CO2 enrichment experiment. Net primary productivity (NPP) of this forest has been enhanced by CO2 enrichment, and the response has been sustained through time. NUE was calculated by dividing annual NPP of each plot by annual N uptake, which was defined as the N content of litter, annual fine root production, annual wood increment, and throughfall minus deposition. NUE was decomposed into its two components: N productivity (AN, NPP divided by plant N content), and mean residence time (MRT) of N in the plant. MRT was calculated as peak content divided by N uptake. Annual N uptake was significantly higher in elevated [CO2], with an average increase over the 6 years of 29%. Most of the difference was in the fine-root component. Uptake accounted for 74% of annual N requirement in ambient [CO2] and a significantly higher fraction (79%) in elevated [CO2], with retranslocation accounting for the remaining fraction of requirement. N uptake and NPP were highly correlated, but NUE was unresponsive to [CO2] because of the offsetting responses of its two components: AN was higher in elevated [CO2], whereas MRT was significantly lower. Higher AN in elevated [CO2] can be explained by the higher rate of photosynthesis per unit leaf N, which is primarily a function of the increased substrate (CO2) supply for photosynthesis rather than a change in N metabolism. The lower MRT in elevated [CO2] was associated with the large fraction of N content in fine roots - tissue which has a relatively short life and from which N is apparently not retranslocated prior to abscission. The possibility of increased NUE providing a mechanism whereby increased NPP can be sustained in elevated CO2 without increased N uptake is not supported in this experiment; hence, either increased N availability or increased access to soil N are required for a sustained productivity response.
Key words: nitrogen-use efficiency, elevated CO2, FACE, Liquidambar styaciflua