Land use reshapes soil cercozoan functional architecture through soil chemistry and fungal networks.

Soil protists are key regulators of microbial turnover and nutrient fluxes, yet their functional organization across managed landscapes remains poorly resolved. Among them, Cercozoa constitute one of the most abundant and ecologically influential lineages in terrestrial ecosystems. However, we lack a mechanistic framework explaining how protist functional composition reorganizes under land-use change and how abiotic filtering and biotic interactions jointly shape these shifts. Here, we examined cercozoan functional trait structure across cropping, pasture and woodland soils in the lower Namoi Valley, New South Wales, Australia, using 18S rRNA gene amplicon sequencing integrated with a curated trait database. Across land uses, communities were dominated by small, gliding bacterivores and omnivores, indicating strong convergence toward highly motile microbial grazers. Cropping soils exhibited higher functional alpha diversity and stronger coupling between trait composition and soil physicochemical gradients than woodland soils, suggesting intensified environmental filtering under agricultural management. Functional diversity was positively associated with fungal diversity and sodium, and negatively associated with organic carbon and total nitrogen, highlighting coordinated responses to both microbial networks and soil chemistry. Several trait categories showed consistent associations with fungal diversity, indicating that biotic coupling contributes to niche structuring alongside abiotic constraints. Together, these results demonstrate that land use reorganizes soil protist functional architecture through interacting physicochemical and biotic drivers, rather than through wholesale biodiversity loss. Trait-based protist metrics therefore provide a mechanistic lens for understanding soil ecological reassembly and offer a promising avenue for integrating microbial eukaryotes into soil health assessment under global change.