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

Josep Ramoneda*, Johannes Le Roux, Stefanie Stadelmann, Emmanuel Frossard, Beat Frey, Hannes Andres Gamper

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Number of pages13
JournalJournal of Applied Ecology
Publication statusE-pub ahead of print - 30 Aug 2021

Bibliographical note

Funding Information:
We thank Dr Peter Manning and two anonymous reviewers for valuable feedback provided on previous drafts of our manuscript. J.R. acknowledges assistance in generating and analysing the and sequence datasets by the Genetic Diversity Center (GDC) and the Functional Genomics Center (FGC), and assistance in processing plant material by Sandra Wenger. J.L.R. acknowledges funding from Macquarie University's Faculty of Science and Engineering and Department of Biological Sciences. H.A.G. is grateful for the funding of the project and former employment at ETH and current employment at FUB. All authors acknowledge assistance by Noel Oettlé, Dr Cecilia Bester, as well as the community of rooibos farmers in the Suid Bokkeveld, organized in the Heiveld Ltd. farmer cooperative. We thank Ruan Kemp for help with translating our abstract into Afrikaans. gryB nodA

Publisher Copyright:
© 2021 The Authors. Journal of Applied Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society.

Copyright 2021 Elsevier B.V., All rights reserved.


  • biological nitrogen fixation
  • community coalescence
  • microbial community assembly
  • plant microbiomes
  • rhizobia
  • rooibos
  • soil mixing
  • symbiosis


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