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.

Progressive nitrogen limitation to ecosystem function in the Duke Forest FACE Experiment.

Finzi, Adrien^*,1, DeLucia, Evan², Lichter, John³, Sinsabaugh, Robert⁴, Schlesinger, William⁵, ¹ Boston University, Boston, MA, USA² University of Illinois, Urbana, IL, USA³ Bowdoin College, Brunswick, ME, USA⁴ University of New Mexico, Albaquerque, NM, USA⁵ Duke University, Durham, NC, USA

ABSTRACT- Experimental increases in the concentration of atmospheric carbon dioxide increase plant growth and net primary production (NPP). In the absence of a concomitant and stoichiometrically balanced increase in N inputs to an ecosystem, the increase in NPP under elevated CO₂ should increase the C:N ratio of the entire ecosystem. This in turn, should lead to a progressive limitation of ecosystem function (e.g. net ecosystem production, NEP) by N. This suggests a short-term response of terrestrial ecosystems to increases in atmospheric CO₂ concentrations. We tested this simple model for ecosystem development using data collected from the first six years of forest growth under elevated CO₂ in the Duke Forest FACE experiment. Initial increases in NPP increased the C:N ratio of the O-horizon and top layers of mineral soil. The increase in the C:N ratio of these horizons appears to have increased microbial-N immobilization as indicated by the lower concentrations of inorganic N in soil extracts and a decrease in the rate of net mineralization. Analyses of extracellular enzyme activities suggest significant microbial-N limitation in the O-horizon. Additionally, there has been a large immobilization of N in woody biomass under elevated CO₂. Ecosystem mass balance shows that the accumulation of N in woody biomass and O-horizon exceeds the rate of N input to the soil system via atmospheric deposition. Collectively these data suggest that 'labile' N pools in soils are being depleted by the increase in the C flux into this ecosystem. Consistent with the progressive N limitation hypothesis, these data predict that down-regulation of NPP should be forthcoming.

Key words: Nitrogen Cycling, Elevated CO2, Net Primary Production, Nutrient Cycling