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

Abstract

Pseudotsuga 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.