Protist predation stimulates peanut productivity by promoting the diversity of rare nitrogen‐fixing bacteria

The root nodule symbiosis between legumes and nitrogen-fixing bacteria (NFB) acts as an important nitrogen source in terrestrial ecosystems. NFB in soil are affected by top-down predation in the food web. However, how protist predation affects abundant and rare sub-communities of NFB remains virtually unknown, limiting the exploitation of soil food webs to promote plant productivity. Here, a 10-yr field experiment combined with a glasshouse experiment was conducted to explore the effects of protist predation on abundant and rare NFB under organic material amendments. Our results revealed that organic material amendments increased the diversity of rare NFB and phagotrophic protists, but decreased the relative abundance of abundant NFB Correlation analysis combined with the glasshouse experiment suggested that protist predation decreased the relative abundance of NFB abundant taxa, but increased the diversity of rare taxa, which further promoted the cytokinin content and decreased the ethylene content in peanut (Arachis hypogaea L.) roots. Subsequent changes in plant hormones regulated the expression of genes involved in rhizobial infection, nodule organogenesis, and bacteroid differentiation, thereby promoting nodulation and increasing peanut yield. Overall, our findings provide unique insights into the interactions between phagotrophic protists and NFB, highlighting their links with plant productivity via predation-stimulated symbiotic nitrogen fixation.