Multidimensional Trait Space of Soil Protists: Ecological Strategies and Environmental Responses.

Soil protists comprise vast eukaryotic diversity and play key roles in microbial food webs, yet their community assembly remains poorly understood from a trait-based perspective. Here, we address this gap by integrating 10 traits related to morphology, behaviour, life history and trophic strategy across broad environmental gradients. RLQ analysis combined with fourth-corner tests revealed two major dimensions of soil protist trait variation. Reduced precipitation drove trait composition towards slower, persistence-oriented strategies, including reduced mobility, narrower niche breadth and increased reliance on asexual reproduction. Nutrient and resource limitation shifted trophic structure away from heterotrophic consumer-related strategies towards autotrophic or mixotrophic energy acquisition. These dimensions were consistently detected in a large-scale field survey and broadly supported by an independent global-change manipulation experiment, indicating that they capture generalizable components of soil protist functional organization. Further functional diversity analyses indicated that environmental gradients primarily constrained the range of occupied trait space rather than consistently reshaping the internal dispersion or evenness of trait distributions. Our study provides a trait-based framework for predicting belowground community assembly and ecosystem functional responses to environmental change.