The bottom-up effects of aridity and the top-down effects of grazing shape biodiversity across trophic levels in the Inner Mongolian grassland

Aridity poses a severe threat to global biodiversity, and grazing further exacerbates ecological stress in drylands. Plants and soil organisms regulate nutrient cycling and ecosystem stability through trophic cascade effects. It remains unclear whether aridity and grazing have consistent effects across trophic levels. We selected 65 paired grazing and ungrazing sites along a natural aridity gradient in the Inner Mongolian grassland in which we explored the diversity of plants, soil bacteria, fungi, nematodes and the impacts of aridity and grazing on trophic cascade effects. Increasing aridity significantly decreased the species richness of plants, pathotrophic fungi, saprotrophic fungi, herbivorous and bacterivorous nematodes. Grazing significantly increased bacterial richness and reduced the richness of herbivorous and omnivorous-predatory nematodes. Overall, aridity exerted a stronger impact on biodiversity compared with grazing. Aridity induced bottom-up effects by initially reducing plant richness, with cascading effects on soil fungal richness and higher trophic communities. Grazing reduced omnivorous-predatory nematode richness, which decreased the richness of lower trophic level organisms through top-down effects. Our research highlights different multi-trophic cascades of aridity and grazing in grasslands, emphasizing the necessity for deeper investigation into biodiversity responses across trophic levels under global change and land use.