Soil tillage promotes predatory and phototrophic protists over parasitic protists

The widespread practice of tillage, which disturbs soil structure and alters its physical and biological properties, may affect protist diversity and functional distribution. In this study, we investigated how tillage, fertilizer, and nitrification inhibitors influence the diversity, community composition, and functional groups of soil protists in a randomized agricultural field experiment comparing conventional tillage with reduced tillage, three nitrification inhibitors, and three fertilizers to test the hypotheses that tillage will decrease overall protist diversity and change protist composition, and that the change in protist community composition will depend on the functional traits, with a relative increase of predators. The genetic diversity of the protist community in 60 soil samples was assessed by DNA metabarcoding using primers targeting the V9 region of the 18S rRNA gene. To infer their ecological role in response to agricultural practices, we assigned trait-based functional diversity to our taxonomic data and found a significant increase in predators and phototrophs while parasites decreased in tilled soil compared to direct-seeded soil. These changes likely reflect bottom-up effects, light availability, and disruption of host-associated material. Fertilizer influenced community structure but not diversity or functional groups, while nitrification inhibitors showed no significant impact. This knowledge will help unravel the factors that shape the functional community structure of protists and will lead to a better understanding of the soil food web. • Tillage was the main driver of the protist community compared to fertilizer, with no detectable effect of nitrification inhibitors. • Tillage increased protist species richness, which contradicts prevailing findings in current literature. • Tillage enhanced functional diversity through higher relative abundances of predators and phototrophs.