Soil fungal alpha diversity is associated with grassland productivity across a natural aridity gradient

Drylands are expanding as a consequence of ongoing climate change, yet the mechanisms through which soil microbiomes are associated with grassland productivity across aridity gradients remain unclear. Here, we characterized bacterial and fungal communities across four sites spanning a natural aridity gradient (from semi-humid to arid) in Inner Mongolia, encompassing meadow, typical, and desert steppes, to assess how microbial diversity, interaction networks, and assembly processes are related to above- and below-ground net primary productivity (NPP). Fungal phylogenetic diversity dropped notably with increasing aridity, decreasing from 200.09 at the semi-humid site to 86.03 at the arid site, representing a 57.0% reduction across the gradient, accompanied by reduced network complexity, intensified deterministic environmental filtering, and an enrichment of drought-tolerant taxa such as Chytridiomycota and Glomeromycota. Concurrently, ANPP and BNPP decreased from 357.99 ± 145.36 and 272.97 ± 70.44 g/m² in meadow steppes to 12.08 ± 8.41 and 18.45 ± 12.75 g/m² in the desert steppe, respectively. In contrast, bacterial diversity remained comparatively stable, although its network stability decreased and community assembly shifted from deterministic to stochastic processes in drier soils. Keystone taxa displayed clear turnover along the aridity gradient, and biomarker analysis identified distinct aridity-responsive bacterial and fungal groups. Fungal diversity, rather than bacterial diversity, showed the strongest positive association with both components of NPP, while aridity exerted strong indirect negative effects by reducing soil nutrients and fungal diversity. Overall, these findings demonstrate that soil fungal diversity is closely associated with grassland productivity under increasing drought conditions, emphasizing the necessity of conserving fungal diversity to maintain ecosystem functioning in a drying world.