PARENT SESSION
Symposium 24: Progressive Nitrogen Limitation of Plant and Ecosystem Response to Rising CO₂
Organized by: Y Luo
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Chatham Ballroom C.

Increased nitrogen uptake supports higher NPP but not carbon storage in CO₂-enriched sweetgum trees.

Norby, Richard^*,1, Jawdy, Sara¹, Ledford, Joanne¹, Sholtis, Johnna², Johnson, Dale³, ¹ Oak Ridge National Laboratory, Oak Ridge, TN² Texas Tech University, Lubbock, TX³ University of Nevada-Reno, Reno, NV

ABSTRACT- The free-air CO₂ enrichment study in a Liquidambar styraciflua (sweetgum) plantation on the Oak Ridge National Environmental Research Park provides the opportunity to evaluate effects of elevated atmospheric CO₂ on nitrogen cycling in an established forest ecosystem in which the N cycle is dominated by recycling. Net primary productivity (NPP) in this stand has increased 24% in response to CO₂ enrichment over 5 years, a consistent response attributable to a sustained increase in photosynthesis and despite an 8% decline in foliar N concentration. Does this increase in photosynthetic nitrogen-use efficiency imply a negative feedback on organic matter production because of the requirement for C and N within relatively constrained proportions? A stand-level analysis of C and N budgets sheds light on this question. Although elevated CO₂ has caused reduced N concentration in both green leaves and leaf litter, there has been no effect of elevated CO₂ on whole-canopy N content or the amount of N returned to soil in leaf litter. NPP increased linearly with N uptake, and N uptake was significantly higher in CO₂-enriched plots. N uptake increased linearly with increasing fine-root length duration, which also was significantly higher in elevated CO₂. However, most of the additional N taken up by trees in CO₂-enriched stands, as well as most of the additional NPP, was used in fine-root production rather than being invested in increased photosynthetic capacity (leaf area duration did not increase) or production of wood (no significant increase after the first year). Hence, there has not been a sustained increase in C or N sequestration in the CO₂-enriched trees compared to trees in ambient air. There also has been no indication that the decomposition rate of leaf or fine root litter has been altered by elevated CO₂, nor has there been any detectable changes in soil microbial N cycling, so the system-level response of accelerated carbon and nitrogen cycling should be sustainable.

Key words: Liquidambar styraciflua, fine-root, FACE, nitrogen