HR: 11:20h
TI: Nitrogen Limitation is Reducing the Enhancement of NPP by Elevated CO2 in a Deciduous Forest
AU: * Norby, R J
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AU: Warren, J M
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AU: Iversen, C M
AF: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AU: Medlyn, B E
AF: School of Biological Sciences, Macquarie University, Sydney, NSW 2019, Australia
AU: McMurtrie, R E
AF: School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia
AU: Hoffman, F M
AF: Computer Science & Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
AB: Accurate model representation of the long-term response of forested ecosystems to elevated atmospheric CO2 concentrations (eCO2) is important for predictions of future concentrations of CO2. For biogeochemical models that predict the response of net primary productivity (NPP) to eCO2, free-air CO2 enrichment (FACE) experiments provide the only source of data for comparison. A synthesis of forest FACE experiments reported a 23% increase in NPP in eCO2, and this result has been used as a model benchmark. Here, we provide new evidence from a FACE experiment in a deciduous forest in Tennessee that N limitation has significantly reduced the stimulation of NPP by eCO2, consistent with predictions from ecosystem and global models that incorporate N feedbacks. The Liquidambar styraciflua stand has been exposed to current ambient atmospheric CO2 or air enriched with CO2 to 550 ppm since 1998. Results from the first 6 years of the experiment indicated that NPP was significantly enhanced by eCO2 and that this was a consistent and sustained response. Now, with 10 years of data, our analysis must be revised. The response of NPP to eCO2 has declined from 24% in 2001-2003 to 9% in 2007. The diminishing response to eCO2 since 2004 coincides with declining NPP in ambient CO2 plots. Productivity of this forest stand is limited by N availability, and the steady decline in forest NPP is closely related to changes in the N economy, as evidenced by declining foliar N concentrations. There is a strong linear relationship between foliar [N] and NPP, and the steeper slope in eCO2 indicates that the NPP response to eCO2 should diminish as foliar N declines. Increased fine-root production and root proliferation deeper in the soil have sustained N uptake, but not to an extent sufficient to benefit aboveground production. The mechanistic basis of the N effect on NPP resides in the photosynthetic machinery. The linear relationships between Jmax and Vcmax with foliar [N] did not change from 1998 to 2008 or in response to eCO2; hence, lower foliar [N] resulted in significant reductions in Jmax, Vcmax, and photosynthesis over time and in eCO2. It is not yet clear whether foliar [N] and NPP will continue to decline or have reached a new steady state indicative of long-term forest response to eCO2. These results are consistent with ecosystem models, which suggest that the NPP response to eCO2 will include a transient increase in NPP followed by a decline to a lower level when fast C and N pools reach quasi-equilibrium at eCO2. Our results also are consistent with optimization models of carbon-water-nitrogen economy, which suggest eCO2 leads to increased fine-root production, declining foliar [N], and diminishing enhancement of aboveground production. At a larger scale, a model incorporating N feedbacks (CLM3-CN) predicts a much smaller enhancement of NPP in eCO2 than the CLM3- CASÁ model without such a feedback. When applied across the terrestrial biosphere, the smaller CO2 fertilization effect has important implications for the pace of climate change.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0469 Nitrogen cycling
DE: 1630 Impacts of global change (1225)
SC: Biogeosciences [B]
MN: 2008 Fall Meeting

Research partially sponsored by the Climate and Environmental Sciences Division (CESD) of the Office of Biological and Environmental Research (OBER), U.S. Department of Energy Office of Science (SC). This research used resources of the National Center for Computational Science (NCCS) at Oak Ridge National Laboratory (ORNL) which is managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.