HR: 0800h
AN: B21D-0910    [Abstracts]
TI: Mycorrhizal Fungi Provide Most of the Nitrogen for Symbiotic Arctic Plants: $^{15}$N Evidence
AU: * Hobbie, J E
EM: jhobbie@mbl.edu
AF: Marine Biological Laboratory, MBL Street, Woods Hole, MA 02543 United States
AU: Hobbie, E A
EM: erik.hobbie@unh.edu
AF: University of New Hampshire, Complex Systems Research Center, Durham, NH 03824 United States
AB: When soil nitrogen is in short supply, most terrestrial plants form symbioses with fungi (mycorrhizae) in which fine hyphal threads take up soil nitrogen, transport it into plant roots, and in return receive plant sugars. Because the transfer rates are very difficult to measure in nature, ecologists need new tools by which to assess the role of mycorrhizal fungi in carbon and nitrogen cycling. Recent studies indicate that the natural abundance of $^{15}$N taken up from the soil by hyphae is changed during transfer of nitrogen to roots; the result is large differences among the natural abundance of $^{15}$N in soil, symbiotic plants, and symbiotic fungi that depend on the mass balance of nitrogen in the mycorrhizal symbiosis. Measurements were carried out in acidic tussock tundra at the Toolik Lake LTER site in Arctic Alaska (68\deg N 149\deg W). The \delta$^{15}$N of soil N was 1.5%, of soil ammonium was 1.5%, of ericoid and ectomycorrhizal plants was -5.0%, and of ectomycorrhizal fungi was 7.0 parts per mille%. The mass balance of the $^{15}$N shows that the plants received 61-86% of their nitrogen from the fungal hyphae. These values, when combined with known plant growth rates, reveal that the plants provided 7-16% of their photosynthetic carbon to the fungi for growth and respiration, or about 25% of all carbon allocated to belowground processes. This analytical technique could be readily applied to other nitrogen-limited ecosystems such as many temperate and boreal forests to quantify the importance for terrestrial carbon and nitrogen cycling of mycorrhizally mediated transfers at the plant-soil interface.
DE: 9315 Arctic region
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting