Aridity gradient overrides degradation in shaping the topsoil microbiome of the Tianshan wild fruit forest

 Highlights

• Aridity index, rather than degradation level, is the paramount driver shaping soil bacterial and fungal communities (explaining ∼43% and ∼30% of the variation, respectively).

• The ecosystem harbors an exceptionally stable core microbiome, with 18,510 bacterial ASVs shared across aridity gradients.

• Dry sub-humid (alternating wet-dry) conditions promote the most complex and stable microbial co-occurrence networks.

• Community assembly of both bacteria and fungi is predominantly governed by stochastic processes (ecological drift).

• Microbial functional profiling reveals nitrogen cycling as the dominant process, with distinct adaptations along the aridity gradient.

Abstract

The Tianshan wild fruit forest, a critical relict ecosystem and biodiversity hotspot, is experiencing severe degradation, a process widely assumed to be the primary driver of negative shifts in soil microbial communities. However, in arid regions, the overarching influence of climatic aridity may supersede localized degradation effects, creating a pivotal scientific question: which factor—aridity or degradation—dominantly regulates the soil microbiome in this unique habitat? To address this, we analyzed 360 topsoil samples across degradation and aridity gradients using high-throughput sequencing. The results demonstrate that the aridity index, not degradation level, is the paramount factor shaping microbial community structure, explaining the largest proportion of variation in both bacterial (∼43%) and fungal (∼30%) communities. The ecosystem harbors an exceptionally stable core microbiome, with community assembly predominantly governed by stochastic processes. Notably, alternating wet-dry (dry sub-humid) conditions significantly enhanced the complexity and stability of microbial co-occurrence networks compared to semi-arid or humid regions. Additionally, soil microbes mediated multiple core ecological processes, with nitrogen cycling as the most abundant—dominated by Rhodoplanes, Alcaligenes, and rhizobial taxa (Bradyrhizobium, Allorhizob), among others—wherein nitrate reduction was exceptionally active in the Tianshan wild fruit forest, particularly in humid and semi-arid habitats. These findings challenge the prevailing degradation-driven paradigm, highlighting aridity as the master regulator of microbial communities. This insight is crucial for guiding conservation strategies, emphasizing that managing water availability and protecting dry sub-humid habitats are essential for maintaining the microbial stability and functional resilience of this invaluable ecosystem under climate change.

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https://www.sciencedirect.com/science/article/pii/S2666517426000416