Soil <scp>pH</scp> thresholds mediate soil biodiversity by regulating interspecific associations in alpine wetlands of the Qinghai–Tibetan Plateau

Abstract Soil pH is recognized as a major determinant of soil biodiversity. However, the extent to which soil pH regulates soil biodiversity by influencing the biotic associations among species is unclear. To address this knowledge gap, we investigated soil multi‐trophic communities—including bacteria, archaea and eukaryotes—from alpine wetland soils spanning a broad pH gradient (4.9–11.1). We observed a unimodal relationship between soil biodiversity and pH, with diversity peaking under moderately alkaline conditions and reaching its lowest levels when soils became extremely alkaline. We further identified a critical pH threshold (pH = 8.82) that corresponded to abrupt shifts in soil multidiversity. Utilizing this threshold, we partitioned the samples into two distinct regimes, enabling the construction and comparison of their multi‐trophic networks. Positive associations were favoured in the pH &lt;8.82 conditions, while networks above the threshold (pH ≥8.82) displayed a significantly greater prevalence of negative associations. Under pH &lt;8.82 conditions, interspecific positive associations support soil biodiversity maintenance. In the highly alkaline and nutrient‐limited conditions, the negative interspecific associations intensify constraints, ultimately driving the observed suppression of soil biodiversity. Collectively, our findings indicate that extremely alkaline environments regulate soil biodiversity not only through direct environmental filtering but also through indirect shifts in positive and negative biotic interactions. Our results offer novel insights into how pH regulates soil biodiversity. Read the free Plain Language Summary for this article on the Journal blog.