N fixation by thin-leaf alder (Alnus tenuifolia) plays a central role in the N economy of interior Alaskan boreal forests, where ecosystem-level N inputs derived from fixation can reach 60 kg N ha^-1 yr^-1.  We are studying mechanisms linking A. tenuifolia growth, N fixation, and mutualist partner choice across early-, mid- and late-successional forests along the Tanana River, where we have previously shown A. tenuifolia to associate with different Frankia partners.  The identity and function of Frankia and ectomycorrhizal partners are being studied in stands where N fixation is naturally up- (high light, low N) and down-regulated (low light, high soil N), and in response to experimental N (down-) and P (up-regulation) fertilization across this same successional sequence.  In 2007, we conducted a pilot study in early-successional floodplain stands to determine whether associations with different Frankia sequence types (FSTs) are related to host cost/benefit tradeoffs driven by varying ecological conditions.  We employed a field system that coupled measurements of nodule respiration and ¹⁵N₂ uptake with subsequent sequence characterization of the same nodules using the non-coding nifD-K IGS region of the Frankia genome.
Results/Conclusions

Nodule respiration and N fixation rates were positively correlated (r²=0.61, P<0.0001) and N fixation costs averaged 9.9 ± 0.7 µmol C respired  µmol N fixed^-1 across the 3 replicate stands (n=150).  A total of 16 unique FSTs were identified, and significant differences in diversity and richness of FSTs were found among stands.  One FST constituted over half the sequences across all nodules and strongly influenced averaged estimates of N fixation cost.  However, of the two next abundant FSTs, one appeared associated with greater average fixation cost, while the other showed higher fixation rates per unit nodule respiration.  Several of the rare FSTs were associated with average N fixation costs, but low N fixation rates. These data suggest that in early-successional stands, host selection for a dominant Frankia sequence type may be related to balancing costs of N fixation rates with high plant C and N demand during a period of rapid growth.