Biogeography of tropical dry forest soils

The tropical dry forest is a tropical biome characterized by prolonged dry seasons, and harbors an exceptionally rich floristic biodiversity. Yet it is among the most threatened biomes and its belowground diversity remains poorly documented. Here, we analyzed the soil environmental DNA with metabarcoding to quantify the belowground diversity and composition of 11 tropical dry forest sites across four biogeographic regions in Colombia, and we provide a comparative analysis of the biogeographic drivers of soil biodiversity, including soil properties, climate, and spatial factors. We found that the soil microbial communities of tropical dry forests had high proportions of taxa associated with dry conditions, such as Actinomycetota, Glomeromycetes, and Amoebozoa, compared to other tropical forests. Soil mesofauna consisted mainly of ants, termites, and earthworms, with few enchytraeids. The soil microbial alpha and beta diversity were primarily driven by soil physicochemical and climatic drivers, bacteria being more sensitive to aridity than fungi. Macroorganisms exhibited patterns of diversity that were less well explained by the tested predictors. Rather, most of their variation was associated with the sampled region, reflecting a marked biogeographic structure and suggesting that macroorganisms are more prone to isolation-by-distance than microorganisms. Overall, the prevalence of drought-associated microbial taxa and the specific composition of soil mesofauna indicate that tropical dry forests harbour a distinctive belowground biota. These findings provide an important baseline for understanding soil biodiversity in tropical dry forests and support conservation and restoration efforts at the biome scale.