Identity and timing of protist inoculation affect plant performance largely irrespective of changes in the rhizosphere microbial community.

Bacterivorous soil protists can have positive impacts on plant performance, making them attractive targets for novel strategies to improve crop production. However, we generally lack the knowledge required to make informed choices regarding the protist species to be used or the optimal conditions for such amendments. Here, we examined how identity, diversity, and timing of inoculation of well-described protists impacted plant development and rhizosphere microbiome assembly. We first studied the impact of individual inoculation of six well-characterized protists on lettuce growth, with IMPORTANCE: The application of microorganisms, including bacterivorous soil protists, has been increasingly suggested as a sustainable agricultural approach. While positive impacts of the presence of predatory protists have been generally reported, the effects of the selected species and amendment conditions are largely unknown. Here, we examined how identity, diversity, and timing of inoculation of well-described protists impacted plant development and rhizosphere microbiome assembly. One species emerged as the one having the strongest impact in our specific system. This result highlights the importance of species selection for optimal outcome, but also suggests a huge potential in the barely investigated protist diversity for targeted application. Furthermore, the application of the inoculants before plant transfer showed the strongest effects on plants, providing some useful and new insights on the optimal time for such amendments.