Nonlinear effects of soil pH on ecosystem multifunctionality along an alpine grassland transect on the northeastern...

Soil acidification driven by atmospheric deposition and anthropogenic reactive nitrogen inputs is a growing threat to biodiversity and ecosystem multifunctionality (EMF). Yet how biodiversity-EMF relationships vary across soil pH gradients remains insufficiently understood, limiting our ability to predict EMF responses to environmental change. Here, we combined field observations from 405 plots at 45 sites along a 430-km soil pH gradient in the northeastern Qinghai-Tibet Plateau to examine nonlinear variation in topsoil multifunctionality and its associations with above- and belowground biodiversity. Across six grassland types and contrasting soil backgrounds, we measured basic topsoil properties, plant richness, soil microbial and ciliate diversity, and multiple ecosystem functions to derive an EMF index. Using linear mixed-effects models, a moving-window approach, and structural equation models, we identified a nonlinear relationship between EMF and soil pH and a shift in the relative importance of biotic correlates across pH regimes. EMF increased from acidic to near-neutral conditions and then declined or became less sensitive to further pH increases, reaching its fitted maximum within a regional functional reference range of approximately pH 6.6-6.8. Around a regional pH threshold of 6.75, plant species richness was the strongest positive correlate of EMF at pH < 6.75, whereas soil biodiversity became more strongly associated with EMF at pH ≥ 6.75. A plant-free soil microcosm experiment manipulating pH across the observed gradient further showed that pH manipulation altered soil biotic diversity and soil respiration under controlled belowground conditions. Our results identify a regional pH threshold in biodiversity-EMF relationships and highlight the value of integrating pH monitoring with plant and soil biodiversity conservation when managing alpine grasslands under climate warming and nitrogen deposition.