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Abstract

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.