Premium
PROGRESSIVE NITROGEN LIMITATION OF ECOSYSTEM PROCESSES UNDER ELEVATED CO 2 IN A WARM‐TEMPERATE FOREST
Author(s) -
Finzi Adrien C.,
Moore David J. P.,
DeLucia Evan H.,
Lichter John,
Hofmockel Kirsten S.,
Jackson Robert B.,
Kim Hyun-Seok,
Matamala Roser,
McCarthy Heather R.,
Oren Ram,
Pippen Jeffrey S.,
Schlesinger William H.
Publication year - 2006
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/04-1748
Subject(s) - ecosystem , mineralization (soil science) , primary production , environmental science , nitrogen cycle , forest ecology , biomass (ecology) , ecology , temperate forest , cycling , biogeochemical cycle , soil water , terrestrial ecosystem , nitrogen , environmental chemistry , chemistry , biology , forestry , geography , organic chemistry
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 CO 2 . The primary mechanism driving this response is a rapid rate of N immobilization by plants and microbes under elevated CO 2 that depletes soils of N, causing slower rates of N mineralization. Under this hypothesis, there is little long‐term stimulation of NPP by elevated CO 2 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 CO 2 enrichment (FACE) experiment. Elevated CO 2 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 CO 2 . In contrast to the PNL hypothesis, microbial‐N immobilization did not increase under elevated CO 2 , 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 CO 2 than ambient CO 2 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 CO 2 ? 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.