Soil Properties and Plant Species Identity Independently Influence Arbuscular Mycorrhizal Fungal Composition Across Lowland Tropical Forests

ABSTRACT Plant species distributions are strongly influenced by soil nutrient availability in tropical forests. Yet, the relative importance of abiotic and biotic factors underlying the composition of pervasive fungal symbionts of plants, arbuscular mycorrhizal fungi (AMF), remains unresolved in lowland tropical forests. Utilizing a long‐term plot network in central Panama with mapped soil properties and tree species distributions, we aimed to understand the relative contribution of soil properties, geographic distance, root traits, and plant species identity in determining AMF community composition. We further asked how plant‐fungal networks vary between sites with contrasting soil phosphorus (P) availability. We sampled fine roots for molecular identification of AMF communities from 140 trees representing 26 species with varied distributions across the soil nutrient availability gradient. We found that plant species identity and soil properties, especially soil P availability and dry‐season moisture deficit, independently structured AMF community composition. Taxonomic turnover and a large number of indicator taxa across a soil P availability gradient provide further evidence for the strong abiotic and biotic structuring of AMF communities. Moreover, a significantly nested plant–AMF network in the low‐P site points to a role of soil nutrient availability in mediating plant–AMF interactions. Our results lay the foundation for future studies to uncover the functional consequences of this symbiosis for plant distributions and ecosystem function in tropical forests.