Influence of soil microbiota in nurse plant systems

Facilitation by nurse plants is a key process involved in the organization of plant communities and maintenance of biodiversity, particularly in harsh environments. Nurse plants increase plant diversity and productivity in these ecosystems, but our knowledge on the mechanisms through which such facilitation operates is still expanding. Despite growing evidence that soil microbiota impact plant fitness and community dynamics, their role in plant facilitation has been little explored. Here, we synthesize available evidence on the effect of nurse plants on the abundance, composition and activity of soil microbial communities, and the effect of these soil communities on beneficiary plant species. Studies conducted mostly in arid and semi-arid systems show that nurse plants promote the development of differentiated soil microbial communities characterized by a higher microbial abundance and activity, the dominance of competitive bacteria and larger mycorrhizal networks, compared to gaps and to coexisting non-nurses. There is also evidence that differentiated soil microbiota associated with nurse plants has positive effects on the establishment, growth and fitness of beneficiary plant species, although the mechanisms involved remain unclear. We suggest that they include increased nutrient availability for plants, a better use of resources through functional complementarity in the microbial community, soil stabilization and also direct molecular signalling between soil microbes and plants that affect plant defence and plant interactions. Evidence for the role of soil microbiota as mediators of facilitation by nurse plants is growing, but there are still too few studies on which to draw generalizable conclusions. Future studies are needed to assess the effect of plant ontogeny and environmental conditions on soil microbial communities under nurse plants and other coexisting species, and to determine the microbial groups and specific mechanisms involved in facilitation by nurse plants.