In long-lived N-fixing plants, environmental conditions affecting plant growth and N demand vary at multiple temporal and spatial scales, and symbiont assemblages on a given host and patterns of allocation to nodule activities have been shown to vary according to environmental factors, suggesting that hosts may alter partner choice and manipulate symbiont assemblages based on shifting plant needs. This study assessed economic trade-offs among N-fixing symbionts of thin-leaf alder (Alnus tenuifolia) by examining whether alder–Frankia associations change in response to the up-regulation (P fertilization) and down-regulation (N-fertilization) of N-fixation activities, and whether these changes are associated with differences among Frankia partners in their relative C cost and/or N benefit to A. tenuifolia.
Relative to control plots, alder in +P plots had significantly higher nodule biomass and N-fixation rates; these parameters were significantly lower in +N plots, translating to stand-level N inputs that were more than an order of magnitude greater in +P than +N plots. Nodule respiration and N-fixation rates were positively correlated, and analyses revealed that alder employs mechanisms to increase the efficiency of C use when N fixation is up-regulated. Of the eight Frankia OTUs (operational taxonomic units) identified, two were dominant, with significant differences in Frankia OTU composition across samples being explained by fertilization treatment. Dominant OTUs had similar up- and down-regulatory responses to treatments but differed in C costs of N fixation, while the most abundant sub-dominant failed to up-regulate N-fixation rates in +P plots. Differences among Frankia OTUs in traits relevant to plant performance may form the basis for host choice and explain successional shifts in alder–Frankia assemblages. We suggest that manipulation of Frankia assemblages is an adaptation for a species such as A. tenuifolia that maintains a high dependence on atmospheric N across radically different growth environments. Because of the strong effects of thin-leaf alder on soil N contents, it appears that host changes in C partitioning to nodule activities, including the physiological consequences of host specificity for Frankia assemblages, results in alder both driving and responding to environmental heterogeneity at small to large spatial scales.