Boreal patterns in the trophic structuring of soil nematodes along climatic and edaphic gradients

Soil nematodes are among the most abundant and diverse soil organisms, occupying multiple trophic positions within detrital soil food webs. Due to their high sensitivity to environmental conditions and close coupling with soil microbial communities, they are recognized as key bioindicators of ecological changes. Here, we provide a large-scale assessment of how climatic and edaphic factors jointly shape nematode trophic structure. Using abundance data from a global network spanning major terrestrial biomes, we examined responses across multiple organizational levels, from individual trophic groups to community-level energy pathways. We integrated univariate generalized additive models, multivariate ordination, and structural equation modelling to quantify these responses. Across trophic groups, soil texture, pH, and vegetation productivity (NDVI) emerged as dominant drivers, highlighting strong bottom-up controls mediated by resource availability and habitat structure. In contrast, absolute soil temperature exerted relatively weak effects, whereas soil–air thermal offset (the empirical difference between soil and air temperature) and aridity played critical roles in regulating higher trophic levels and functional energy pathways. Predatory nematodes and the Nematode Channel Ratio (NCR) were particularly sensitive to microclimatic buffering, indicating that thermal decoupling between soil and air captures ecologically meaningful constraints not reflected by the commonly-used macroclimatic averages. Overall, for the first time we show the extent to which patterns of the soil nematode trophic organization are governed by the interplay between edaphic constraints, resource availability, and soil microclimate. Incorporating belowground thermal conditions alongside traditional variables is therefore essential for understanding and predicting soil food-web responses to future global changes.