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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
Citation:
Hobbie, E. A., D M. Olszyk, P T. Rygiewicz, D T. Tingey, AND M G. Johnson. 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. TREE PHYSIOLOGY. Heron Publishing, Victoria, B.C, Canada, 21:1113-1122, (2001).
Description:
In an experiment using Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings and a 2x2 factorial design in enclosed mesocosms, temperatures were maintained at ambient or +3.5 degrees C above ambient, and CO2 levels were maintained at ambient or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured levels of mycorrhizal infection, foliar nitrogen (N) levels, and d15N signatures in needles of different age classes from Summer 1993 through Summer 1997. After an initial colonization phase, mycorrhizal fungi had colonized nearly all root tips across all treatments by Spring 1994. Elevated CO2 lowered folair %N and elevated temperatures increased foliar %N. However, CO2 generally did not affect N levels on an areal basis. Foliar d15N dropped from -1 o/oo initially to between -4 o/oo and -7 o/oo at the final harvest. We attribute these declines to transfer of isotopically depleted N from mycorrhizal fungi. By using the equation Tr = 1 + (d15N - d15Navailable N)Af, where Tr is the mycorrhizal transfer ratio and Af the isotopic fractionation during transfer (10 o/oo), we estimated that from 33% to 61% of N taken up by mycorrhizal fungi was transferred to host plants. For 1993-94 we combined isotopic and N concentration data to calculate fluxes of N for first- and second-year needles. Influxes increased for second-year needles at the elevated temperature compared with the ambient temperature, probably because of enhanced soil mineralization under elevated temperatures. Yearly efflux and influx in second-year needles were similar and about 5-% of leaf N, whereas the equivalent fluxes in first-year needles were 50% and 160% respectively, indicated greater sink strength for younger needles. These results indicate that d15N measurements can improve our understanding of N fluxes within plants, of plant-mycorrhizal partitioning of N, and of plant responses to climatic changes.