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

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.