LAUSR

ABSTRACT Parasitic protists such as Apicomplexa, an abundant group of soil protists, contribute to ecosystem processes and nutrient cycling in belowground soil systems through their obligate symbioses with soil Metazoa. Yet despite the importance of soil parasites, the biodiversity and biogeography of Apicomplexa in belowground systems remain poorly characterized. Leveraging 205 soils collected across a rainfall gradient spanning the isthmus of Panama, we sought to understand the distribution of soil Apicomplexa lineages and how abiotic (e.g., soil and climatic) and biotic (e.g., soil Metazoa) factors relate to their diversity and structure. Apicomplexa were highly heterogeneous across the samples and comprised 30% of the soil protist community on average. Soil pH, along with phosphorus and magnesium, best explained the overall distribution of Apicomplexa. Soil Metazoa distributions also corresponded to Apicomplexa distributions, and many Metazoan taxa co‐occurred with particular Apicomplexa, which may reflect ecological interactions (such as parasitism) or shared habitat preferences. These results highlight the potential roles of both soil and climatic variables and putative hosts in structuring parasite distributions in belowground tropical systems. Our work builds a broader understanding of Apicomplexa biodiversity in tropical soils and sheds light on environmental factors that may contribute to shaping their distribution in belowground systems. These results help inform our understanding of the importance of parasites in tropical forest soils.