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

Area of Science:

• Ecology
• Biogeochemistry
• Climate Change Science

Background:

• The progressive nitrogen limitation (PNL) hypothesis predicts declining net primary production (NPP) in ecosystems with elevated atmospheric CO2 due to nitrogen (N) immobilization.
• This occurs as plants and microbes rapidly sequester N, depleting soil N and slowing N mineralization, thus limiting long-term NPP stimulation without external N inputs.

Purpose of the Study:

• To test the PNL hypothesis using data from the Duke Forest free-air CO2 enrichment (FACE) experiment.
• To evaluate the impact of elevated CO2 on nitrogen pools, fluxes, and ecosystem C:N ratios over six years.

Main Methods:

• Analysis of carbon (C) and nitrogen (N) pools and fluxes in tree biomass, microbes, and soils.
• Utilized data from the Duke Forest free-air CO2 enrichment (FACE) experiment spanning 1997-2002.

Main Results:

• Elevated CO2 significantly increased NPP by 18-24% in the initial six years.
• Nitrogen immobilization increased in tree biomass and the O horizon under elevated CO2, supporting part of the PNL hypothesis.
• Microbial N immobilization did not increase, and the decline in net N mineralization was not significantly faster under elevated CO2.
• Ecosystem C:N ratios widened faster under elevated CO2, suggesting delayed PNL due to enhanced C fixation relative to N.

Conclusions:

• While elevated CO2 initially enhances forest NPP, increased N immobilization in biomass and soil indicates potential for progressive nitrogen limitation.
• Widening ecosystem C:N ratios and N accrual may counteract or delay PNL, making the long-term outcome uncertain.
• Definitive answers on whether NPP will decline require continued long-term observation of these ecosystem processes.