Keystone predatory protists are associated closely with ammonia-oxidizing microorganisms in an acidic Ultisol

Predatory protists are widely recognized as critical biotic forces driving soil microbial communities, but their top-down controls on ammonia-oxidizing microorganisms (AOMs), the major players in nitrification, are largely unresolved. Here, we investigated the communities of predatory protists and their associations with AOMs using high-throughput sequencing and network analysis in soil aggregates following various long-term organic substitutions. We found that organic substitutions increased while soil aggregation decreased the alpha diversity of predatory protists. Predatory protistan communities were significantly associated with AOMs. Variosea, an important group of Amoebozoa, were the keystone predatory protists associated with the AOMs. Collectively, our findings highlight the importance of predatory protists, especially Variosea, in regulating the communities of AOMs in an acidic Ultsisol, with implications for managing nitrification by predatory protists in agricultural soils.