Document: YIQ-3-59-8

Progressive nitrogen limitation of plant and ecosystem responses to elevated CO₂: A reexamination.

LUO, Y.*

University of Oklahoma, Norman, OK 73019, USA ¹

Abstract:
Ecosystem models that link nutrient cycling with plant production generally predict long-term downregulation in plant production caused by progressive nitrogen (N) constraints in elevated CO₂. The concept is that increased carbon (C) influx under elevated CO₂ will immobilize more N in plant biomass and soil organic matter, progressively leading to less N available for plant uptake. However, the predicted downregulation in production has not been confirmed by most of the field CO₂ experiments. The discrepancy between model predictions and experimental data apparently results from a fundamental knowledge gap concerning ecosystem C and N interactions. This study examines N demand and supply mechanisms in the context of Duke Forest FACE site where the CO₂ concentration has been increased from the ambient level to 560 ppm since 1996. In response to elevated CO₂, both measurements and modeling indicated an approximately 40% increase in ecosystem C influx and a 300 g C m^-2 yr^-1 increase of net ecosystem productivity (NEP). The latter would require an additional 6 g N m^-2 yr^-1 to balance C:N relationships in the ecosystem. A model is also used to explore various N supply mechanisms that might meet N demand in response to rising CO₂. Those mechanisms include adjustment in the C:N ratio in live biomass, net mineralization, and rhizosphere expansion, N fixation by free living organisms and symbiotic species, N deposition, reduced losses through leaching and volatilization.

Keywords: Global change, forest, modeling, nitrogen mineralization, productivity

This abstract is being presented at: 11:00 AM in session:
Oral Session #40: Elevated CO₂ In Forest Systems.