# Composition of soil Frankia assemblages across ecological drivers parallels that of nodule assemblages in Alnus incana ssp. tenuifolia in interior Alaska

## TL;DR

This study explores how soil and nodule assemblages of nitrogen-fixing bacteria called Frankia vary in relation to host plants and environmental factors in Alaska.

## Contribution

The study reveals how host plants and environmental changes shape the diversity and composition of Frankia assemblages in both soil and plant nodules.

## Key findings

- Soil Frankia assemblages showed greater phylogenetic diversity compared to nodule assemblages.
- Host plant presence increased genotypes specific to Alnus incana ssp. tenuifolia and decreased those specific to another Alnus species.
- Fertilization with nitrogen and phosphorus unexpectedly increased the proportion of symbiotic Frankia genotypes.

## Abstract

In root nodule symbioses (RNS) between nitrogen (N)‐fixing bacteria and plants, bacterial symbionts cycle between nodule‐inhabiting and soil‐inhabiting niches that exert differential selection pressures on bacterial traits. Little is known about how the resulting evolutionary tension between host plants and symbiotic bacteria structures naturally occurring bacterial assemblages in soils. We used DNA cloning to examine soil‐dwelling assemblages of the actinorhizal symbiont Frankia in sites with long‐term stable assemblages in Alnus incana ssp. tenuifolia nodules. We compared: (1) phylogenetic diversity of Frankia in soil versus nodules, (2) change in Frankia assemblages in soil versus nodules in response to environmental variation: both across succession, and in response to long‐term fertilization with N and phosphorus, and (3) soil assemblages in the presence and absence of host plants. Phylogenetic diversity was much greater in soil‐dwelling than nodule‐dwelling assemblages and fell into two large clades not previously observed. The presence of host plants was associated with enhanced representation of genotypes specific to A. tenuifolia, and decreased representation of genotypes specific to a second Alnus species. The relative proportion of symbiotic sequence groups across a primary chronosequence was similar in both soil and nodule assemblages. Contrary to expectations, both N and P enhanced symbiotic genotypes relative to non‐symbiotic ones. Our results provide a rare set of field observations against which predictions from theoretical and experimental work in the evolutionary ecology of RNS can be compared.

N‐fixing host plants can theoretically affect the structure of soil‐dwelling assemblages of symbiotic microbes via selective amplification of specific genotypes. However, little is known about how soil‐dwelling and nodule‐dwelling assemblages compare in natural populations. This study examined this comparison in the Alnus‐Frankia system across a primary successional gradient in interior Alaska.