title: Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest.
abstract: 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 (NA) 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.
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Tennessee 37831-6422, USA. rjn@ornl.gov
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Norby
authors.forename: Colleen M
authors.initials: CM
authors.lastname: Iversen
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D018533:Biomass; D002245:Carbon Dioxide / Q000378:metabolism / Q000502:physiology*; D017753:Ecosystem; D031317:Liquidambar / Q000254:growth & development* / Q000378:metabolism*; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000737:chemistry / Q000254:growth & development; D018517:Plant Roots / Q000737:chemistry / Q000254:growth & development; D012621:Seasons; D013997:Time Factors; D014197:Trees / Q000254:growth & development / Q000378:metabolism*
chemical_list: D002245:Carbon Dioxide; D009584:Nitrogen