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Abstract

seudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 × 2 factorial design in enclosed mesocosms at ambient temperature or 3.5 °C above ambient, and at ambient CO₂ concentration ([CO₂]) or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured the extent of mycorrhizal infection, foliar nitrogen (N) concentrations on both a weight basis (%N) and area basis (Narea), and foliar δ¹⁵N signatures (¹⁵N/¹⁴N ratios) from summer 1993 through summer 1997. Mycorrhizal fungi had colonized nearly all root tips across all treatments by spring 1994. Elevated [CO₂] lowered foliar %N but did not affect Narea, whereas elevated temperature increased both foliar %N and Narea. Foliar δ¹⁵N was initially –1‰ and dropped by the final harvest to between –4 and –5‰ in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar δ¹⁵N among treatments, we conclude that mycorrhizal fungi had similar N allocation patterns across CO₂ and temperature treatments. We combined isotopic and Narea data for 1993–94 to calculate fluxes of N for second- and third-year needles. Yearly N influxes were higher in second-year needles than in third-year needles (about 160 and 50% of initial leaf N, respectively), indicating greater sink strength in the younger needles. Influxes of N in second-year needles increased in response to elevated temperature, suggesting increased N supply from soil relative to plant N demands. In the elevated temperature treatments, N effluxes from third-year needles were higher in seedlings in elevated [CO₂] than in ambient [CO₂], probably because of increased N allocation below ground. We conclude that N allocation patterns shifted in response to the elevated temperature and [CO₂] treatments in the seedlings but not in their fungal symbionts.