Ecosystem‐level consequences of symbiont partnerships in an N‐fixing shrub from interior Alaskan floodplains

Abstract

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 alderFrankia associations change in response to the up-regulation (P fertilization) and downregulation (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 þP plots had significantly higher nodule biomass and Nfixation rates; these parameters were significantly lower in þN plots, translating to stand-level N inputs that were more than an order of magnitude greater in þP than þN 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 þP 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.

Novel bacterial lineages associated with boreal moss species

“…To quantify field rates of N 2 ‐fixation for each sample, we used an isotopic enrichment approach modified from Ruess et al . (). Each sample was placed in a 60 ml translucent polycarbonate syringe that was depressed to contain 10 ml of air.…”

Riparian Corridors: A New Conceptual Framework for Assessing Nitrogen Buffering Across Biomes

“…or Dryas sp.) often dominates floodplains, increasing carbon and nitrogen availability simultaneously, as water moves downstream (Ruess et al, 2013;Taylor, 2013;Malone et al, 2018). Furthermore, headwater aquatic primary production (algae) is often phosphorus limited, resulting in rapid nitrification of what little ammonium makes it to the stream channel, though nitrogen demand increases as rivers meet the sea McClelland et al, 2012).…”

Light availability and rhizobium variation interactively mediate the outcomes of legume–rhizobium symbiosis

“…Ruess et al. () estimated nodule respiration, N fixation, and Frankia strain identity in a field survey of Alnus tenuifolia and found that Frankia strains vary in terms of both N fixation and respiratory cost. Some inocula in our study seem to be low quality in terms of N fixation ability (based on C:N and δ 15 N, relative to other inocula), resulting in N‐limited plants independent of C availability, while others appeared to fix more N and thus allow hosts to respond positively to increased C availability, though for some (262) only in ambient light, potentially because of substantial C costs.…”

“…Rhizobium strains are well known to vary in partner quality, which is most often measured as relative plant growth and fitness (reviewed by Denison, 2000;Simms and Taylor, 2002;Heath and Stinchcombe, 2014;Sachs et al, 2018). These growth and fitness benefits to the plant depend on both the benefits and costs of symbiosis, i.e., the benefits of fixed N received minus the C costs including nodule formation, nodule respiration, and production of the bacterial storage compound poly-3-hydroxybutyrate (PHB) (Tjepkema and Winship, 1980;Minchin and Witty, 2005;Ratcliff et al, 2008;Ruess et al, 2013). Plant nutrient status and light interactively determine plant growth (reviewed by Elser et al, 2010); therefore, genetic variation in N-fixing rhizobium symbionts may interact with light availability to influence the outcome of mutualism for plant hosts.…”