Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest

Journal Article · Sun Jan 01 04:00:00 EST 2006 · Ecology

DOI:https://doi.org/10.1890/04-1950· OSTI ID:932148

Norby, Richard J ^[1]; Iversen, Colleen M ^[1]

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1. ORNL

The Progressive Nitrogen Limitation (PNL) hypothesis suggests that ecosystems in a CO2-enriched atmosphere will sequester C and N in long-lived biomass and soil organic pools, thereby limiting available N and constraining the continued response of net primary productivity to elevated [CO2]. Here, we present a six-year record of N dynamics of a sweetgum (Liquidambar styraciflua) stand exposed to elevated [CO2] in the free-air CO2 enrichment (FACE) experiment at Oak Ridge, Tennessee, USA. We also evaluate the concept of PNL for this ecosystem from the perspective of N uptake, content, distribution, and turnover, and N-use efficiency. Leaf N content was 11% lower on a leaf mass basis (NM) and 7% lower on a leaf area basis (N{sub A}) in CO2-enriched trees. However, there was no effect of [CO2] on total canopy N content. Resorption of N during senescence was not altered by [CO2], so NM of litter, but not total N content, was reduced. The NM of fine roots was not affected, but the total amount of N required for fine-root production increased significantly, reflecting the large stimulation of fine-root production in this stand. Hence, total N requirement of the trees was higher in elevated [CO2], and the increased requirement was met through an increase in N uptake rather than increased retranslocation of stored reserves. Increased N uptake was correlated with increased net primary productivity (NPP). N-use efficiency, however, did not change with CO2 enrichment because increased N productivity was offset by lower mean residence time of N in the trees. None of the measured responses of plant N dynamics in this ecosystem indicated the occurrence of PNL, and the stimulation of NPP by elevated [CO2] was sustained for the first six years of the experiment. Although there are some indications of developing changes in the N economy, the N supply in the soil at this site may be sufficient to meet an increasing demand for available N, especially as the roots of CO2-enriched trees explore deeper in the soil profile.

• Research Organization: Oak Ridge National Laboratory (ORNL); Oak Ridge National Environmental Research Park
• Sponsoring Organization: SC USDOE - Office of Science (SC)
• DOE Contract Number: AC05-00OR22725
• OSTI ID: 932148
• Journal Information: Ecology, Journal Name: Ecology Journal Issue: 1 Vol. 87; ISSN 0012-9658; ISSN ECOLAR
• Country of Publication: United States
• Language: English

Related Subjects

09 BIOMASS FUELS
BIOMASS
CARBON DIOXIDE
DISTRIBUTION
ECOSYSTEMS
EFFICIENCY
FORESTS
HYPOTHESIS
NITROGEN
PRODUCTION
PRODUCTIVITY
SOILS
STIMULATION
TREES