Foliar nitrogen concentrations and natural abundance of (15)N 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 degrees 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 delta15N 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 N(area), whereas elevated temperature increased both foliar %N and Narea. Foliar delta15N was initially -1 per thousand and dropped by the final harvest to between -4 and -5 per thousand in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar delta15N 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.
Hobbie
E A
EA
National Research Council, US Environmental Protection Agency, Corvallis, OR 97333, USA.
Olszyk
D M
DM
Rygiewicz
P T
PT
Tingey
D T
DT
Johnson
M G
MG
D002245:Carbon Dioxide / Q000502:physiology; D005658:Fungi / Q000502:physiology; D009584:Nitrogen / Q000032:analysis* / Q000378:metabolism; D009587:Nitrogen Isotopes / Q000032:analysis / Q000378:metabolism; D018515:Plant Leaves / Q000737:chemistry* / Q000502:physiology; D018517:Plant Roots / Q000382:microbiology / Q000502:physiology; D028224:Pseudotsuga / Q000737:chemistry / Q000382:microbiology / Q000502:physiology*; D013559:Symbiosis / Q000502:physiology; D013696:Temperature
D009587:Nitrogen Isotopes; D002245:Carbon Dioxide; D009584:Nitrogen