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Nitrogen availability and colonization by mycorrhizal fungi correlate with nitrogen isotope patterns in plants


Hobbie,  E. A.
Department Biogeochemical Systems, Prof. D. Schimel, Max Planck Institute for Biogeochemistry, Max Planck Society;

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Hobbie, E. A., & Colpaert, J. V. (2003). Nitrogen availability and colonization by mycorrhizal fungi correlate with nitrogen isotope patterns in plants. New Phytologist, 157(1), 115-126. doi:10.1046/j.1469-8137.2003.00657.x.

Nitrogen isotope (δ 15 N) patterns in plants may provide insight into plant N dynamics. Here, two analytical models of N-isotope cycling in plants and mycorrhizal fungi were tested, as dominant plants in many forest ecosystems obtain most of their N through intereactions with mycorrhizal fungi. Fungi were treated either as a single well-mixed N pool, or as two N pools (one available, plus one not available, for transfer to the host). Models were compared against complete biomass and 15 N budgets from culture studies of nonmycorrhizal and ectomycorrhizal Pinus sylvestris (colonized with Suillus luteus or Thelephora terrestris ) grown exponentially at low and high N supply. Fungal biomass and N increased at low N relative to high N supply, whereas needle δ 15 N decreased. Needle δ 15 N correlated strongly and negatively with biomass of extraradical hyphae. Our data and models suggest that low plant δ 15 N values in low productivity and N-limited environments result partly from high retention of 15 N-enriched N by mycorrhizal fungi; this retention was driven by increased C flux to fungi under N-limited conditions. The two-pool model of fungal N accounted for greater variability in plant δ 15 N than the one-pool model. Plant δ 15 N patterns may indicate relative allocation of fixed C from plants to mycorrhizal fungi under some conditions. Studies are needed on whether patterns observed in culture can be applied to interpret field measurements of δ 15N.