title: Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest.
abstract: 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 CO2. 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.
authors.affiliation: Department of Biology, Boston University, Massachusetts 02215, USA. afinzi@bu.edu
authors.forename: Adrien C
authors.initials: AC
authors.lastname: Finzi
authors.forename: David J P
authors.initials: DJ
authors.lastname: Moore
authors.forename: Evan H
authors.initials: EH
authors.lastname: DeLucia
authors.forename: John
authors.initials: J
authors.lastname: Lichter
authors.forename: Kirsten S
authors.initials: KS
authors.lastname: Hofmockel
authors.forename: Robert B
authors.initials: RB
authors.lastname: Jackson
authors.forename: Hyun-Seok
authors.initials: HS
authors.lastname: Kim
authors.forename: Roser
authors.initials: R
authors.lastname: Matamala
authors.forename: Heather R
authors.initials: HR
authors.lastname: McCarthy
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Pippen
authors.forename: William H
authors.initials: WH
authors.lastname: Schlesinger
mesh_terms: D001419:Bacteria / Q000737:chemistry; D018533:Biomass; D002245:Carbon Dioxide / Q000502:physiology*; D017753:Ecosystem; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D012987:Soil / Q000032:analysis; D012988:Soil Microbiology; D013997:Time Factors; D014197:Trees / Q000737:chemistry / Q000254:growth & development* / Q000378:metabolism*; D014934:Wood
chemical_list: D012987:Soil; D002245:Carbon Dioxide; D009584:Nitrogen