Soil microbial community coalescence and fertilization interact to drive the functioning of the legume–rhizobium symbiosis

1. Soil microbial community coalescence, where entire microbial communities mix and interact under new conditions, is a widespread phenomenon whose applicability for targeted root microbiome assembly has not been studied. Whether soil mixing can lead to predictable outcomes for community assembly and functioning of specific functional groups, for example, N₂-fixing rhizobia, remains unknown.

2. Using a legume shrub adapted to nutrient-poor soils, we tested the effects of community coalescence on plant nutrition and growth, and its influence on the rhizobial root nodule symbiosis. We grew seedlings of rooibos [Aspalathus linearis (Burm.f.) Dahlg.] in individual rhizosphere soils collected from cultivated and wild rooibos plants and their mixtures, and included a fertilization treatment. Portions of the taxonomic gyrB and the symbiotic nodA gene makers were sequenced to characterize rhizobial communities present in rooibos root nodules under the different soil conditions.

3. Overall, community coalescence by soil mixing had positive effects on plant nutrition and growth, and interacted with fertilizer addition to concurrently change rhizobial taxonomic evenness and promote higher relative N₂ fixation. We identified particular rhizobia preferentially associated with rooibos plants that showed higher N fractions from N₂ fixation, raised in mixed fertilized soils.

4. These findings indicate that soil bacterial community coalescence and fertilization can have synergistic effects on plant performance, while promoting the assembly of alternative symbiotic rhizobial communities that provide improved nutritional benefits to host plants.

5. Synthesis and applications. The combination of soil mixing and fertilizer addition may be an important, but hitherto overlooked measure to improve the functioning of rhizobium symbioses in legume crops. Microbial community coalescence should gain recognition as a potentially effective mechanism to improve the functioning of plant microbiomes.