Drivers of soil biodiversity vary with organism type along an extensive aridity gradient

Soils harbor a diverse range of biodiversity, including microbes and soil animals, which are crucial in supporting ecosystem functions. Despite the well-known effects of biotic and abiotic factors on soil biodiversity, their relative importance on different soil organisms remains less known, reducing our ability to maintain multiple soil communities under environmental changes. We sampled 150 sites from humid to arid areas in eastern Australia to explore the direct and indirect effect of climate (aridity, temperature), soil properties (pH, texture, infiltrability), plants (woody structure and plant richness) and soil surface attributes (litter, biocrusts, surface morphology) on overall soil biodiversity and the richness of bacteria, fungi, invertebrates and protists. We found that the relative importance of biotic and abiotic attributes varied with soil organisms and specific phylum, with bacterial richness related to soil pH, total plant richness and surface stability, fungal richness associated with litter, invertebrate richness related to aridity and total plant richness, and protist richness associated with soil pH. Larger tree canopies, greater tree spacing and increasing aridity either suppressed (bacteria, fungi, protists) or enhanced (invertebrates) richness indirectly by either exacerbating the negative effect of litter depth and soil pH, or promoting the positive effect of groundstorey foliage cover. Our study provides empirical evidence of how different soil organisms respond to environmental changes, indicating trade-offs among soil communities with no single environmental condition can maximum soil biodiversity. Moreover, our results highlight that increasing dryness would results in fewer bacterial and invertebrate species, potentially leading to declines in soil biodiversity and ecosystem functions under the predicted hotter and drier climate.