Abstract: Tibetan alpine meadow degradation profoundly alters plant-microbe interactions and carbon stability, yet its impacts on host-arbuscular mycorrhizal (AM) fungal symbiosis and carbon allocation in the hyphosphere remain unclear. Here, we employed high-throughput sequencing of the 18S rRNA gene to investigate plant-AM fungal preferences across nondegraded, moderately degraded and severely degraded alpine meadows on the Tibetan Plateau. We further utilized compartmentalized microcosms under greenhouse conditions to investigate the impact of alpine meadow degradation on carbon allocation in the hyphosphere. Our findings revealed that alpine meadow degradation significantly increased bacterial biomass while decreasing fungal biomass in the hyphosphere of dominant plants. Moreover, the bacteria-to-fungi biomass ratio in the hyphosphere of dominant and companion plants in severely degraded alpine meadows was significantly higher by 184.9% and 19.9%, respectively, compared with nondegraded alpine meadows. Host-AM fungi preference analyses demonstrated strong host-AM fungal specificity in nondegraded alpine meadows, which was lost with degradation, coinciding with increased AM fungal diversity in degraded alpine meadows. Twenty-two of the 46 most abundant AM fungal operational taxonomic units (47.8%) showed significant host preference, mostly from Glomeraceae and Claroideoglomeraceae. These findings highlight that alpine meadow degradation restructures plant-AM fungal-bacterial interactions, disrupting plant-AM fungal preference and allocating more carbon to hyphosphere bacteria, potentially as an adaptive strategy to environmental stress and sustain nutrient acquisition. Our study provides critical insights for guiding grassland restoration by leveraging plant-AM fungal mutualistic symbiosis and microbial carbon allocation tradeoffs in degraded ecosystems.

Key words: arbuscular mycorrhizal fungi, hyphosphere, carbon allocation, host-microbial preference, alpine meadow degradation