Soil rhizobia promote plant yield by increasing tolerance to pests and pathogens under field conditions

Summary

Nitrogen-fixing, root-colonizing rhizobia are abundant soil microbes that form mutualisms with legumes. Rhizobia provide direct benefits to hosts by fixing nitrogen and promoting nutrient acquisition. However, whether rhizobia indirectly alter plant yield by affecting insect pests and insect-borne pathogens is less well understood, with conflicting results from existing lab studies. Here we used a field experiment to test whether effects of rhizobia on plants extended beyond nitrogen provisioning to include greater tolerance to aphids and pathogens. Specifically, we manipulated field soil with four treatments: (i) untreated, (ii) sterilized, (iii) sterilized with nitrogen fertilizer, and (iv) sterilized with rhizobia; we then assessed the direct effects on plant yields as well as tolerance