Foliar nitrogen concentrations and natural abundance of N-15 suggest nitrogen allocation patterns of Douglas-fir and mycorrhizal fungi during development in elevated carbon dioxide concentration and temperature

Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 x 2 factorial design in enclosed mesocosms at ambient temperature or 3. 5 degreesC above ambient, and at ambient CO2 concentration ([CO2]) or 179 p pm above ambient. Two additional mesocosms were maintained as open controls . We measured the extent of mycorrhizal infection, foliar nitrogen (N) conc entrations on both a weight basis (%N) and area basis (Narea), and foliar d elta N-15 signatures (N-15/N-14 ratios) from summer 1993 through summer 199 7. Mycorrhizal fungi had colonized nearly all root tips across all treatmen ts by spring 1994. Elevated [CO2] lowered foliar %N but did not affect N-ar ea, whereas elevated temperature increased both foliar %N and Nar, Foliar d elta N-15 was initially -1 parts per thousand and dropped by the final harv est to between -4 and -5 parts per thousand in the enclosed mesocosms, prob ably because of transfer of isotopically depleted N from mycorrhizal. fungi . Based on the similarity in foliar delta N-15 among treatments, we conclud e that mycorrhizal fungi had similar N allocation patterns across CO2 and t emperature treatments. We combined isotopic and N-area 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 a nd 50% of initial leaf N, respectively), indicating greater sink strength i n the younger needles. Influxes of N in second-year needles increased in re sponse to elevated temperature, suggesting increased N supply from soil rel ative to plant N demands. In the elevated temperature treatments, N effluxe s from third-year needles were higher in seedlings in elevated [CO2] than i n ambient [CO2], probably because of increased N allocation below ground. W e conclude that N allocation patterns shifted in response to the elevated t emperature and [CO2] treatments in the seedlings but not in their fungal sy mbionts.