Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest

Adrien C. Finzi, David J.P. Moore, Evan H. DeLucia, John Lichter, Kirsten S. Hofmockel, Robert B. Jackson, Hyun Seok Kim, Roser Matamala, Heather R. McCarthy, Ram Oren, Jeffrey S. Pippen, William H. Schlesinger

Research output: Contribution to journalArticlepeer-review


A hypothesis for progressive nitrogen limitation (PNL) proposes that net primary production (NPP) will decline through time in ecosystems subjected to a step-function increase in atmospheric CO2. The primary mechanism driving this response is a rapid rate of N immobilization by plants and microbes under elevated CO2 that depletes soils of N, causing slower rates of N mineralization. Under this hypothesis, there is little long-term stimulation of NPP by elevated CO2 in the absence of exogenous inputs of N. We tested this hypothesis using data on the pools and fluxes of C and N in tree biomass, microbes, and soils from 1997 through 2002 collected at the Duke Forest free-air CO2 enrichment (FACE) experiment. Elevated CO2 stimulated NPP by 18-24% during the first six years of this experiment. Consistent with the hypothesis for PNL, significantly more N was immobilized in tree biomass and in the O horizon under elevated CO2. In contrast to the PNL hypothesis, microbial-N immobilization did not increase under elevated CO2, and although the rate of net N mineralization declined through time, the decline was not significantly more rapid under elevated CO 2. Ecosystem C-to-N ratios widened more rapidly under elevated CO2 than ambient CO2 indicating a more rapid rate of C fixation per unit of N, a processes that could delay PNL in this ecosystem. Mass balance calculations demonstrated a large accrual of ecosystem N capital. Is PNL occurring in this ecosystem and will NPP decline to levels under ambient CO2? The answer depends on the relative strength of tree biomass and O-horizon N immobilization vs. widening C-to-N ratios and ecosystem-N accrual as processes that drive and delay PNL, respectively. Only direct observations through time will definitively answer this question.

Original languageEnglish (US)
Pages (from-to)15-25
Number of pages11
Issue number1
StatePublished - Jan 2006


  • Elevated CO
  • Net primary production
  • Nitrogen cycling
  • Temperate forest

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics


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