Wetland to grassland transition in alpine ecosystems alters microbial network complexity and multifunctionality

Alpine wetland ecosystems store disproportionate amounts of global soil carbon and provide critical ecosystem services, yet widespread conversion to grasslands threatens their stability and functioning. Despite recognition of this global phenomenon, a mechanistic understanding of how grasslandification-the conversion of sedge-dominated wetlands to grass-or-forb dominated systems-affects belowground microbial communities remains limited. Here, we investigated soil bacterial and fungal community responses to grasslandification across a degradation gradient in alpine wetland meadows of the Qinghai-Tibetan Plateau (QTP), representing the world's largest high-altitude wetland complex. We analyzed 108 soil samples from two depths (0-10 and 10-20 cm) across three ecosystem states: undisturbed alpine wetland meadow, intermediate alpine meadow, and degraded meadow. Grasslandification restructured belowground communities through three cascading mechanisms. First, bacterial assembly was predominantly stochastic (>90% across sites), with subtle shifts toward undominated and heterogenous selection in degraded surface soil. In contrast, fungi showed deterministic assembly (homogenous selection ∼49.7%) in the degraded meadow, indicating intensified environmental filtering under habitat degradation. Second, microbial network architecture exhibited depth-dependent simplification, with surface soils showing systematic declines in connectivity and increased modularity from wetland to degraded meadow, while deeper soils displayed contrasting patterns of enhanced bacterial linkage density. Third, soil multifunctionality (SMF) declined significantly along the degradation gradient, driven primarily by losses in soil organic carbon (43% decline), total nitrogen (23%), and total phosphorus 38%). Structure equation modeling revealed that soil properties mediated 71% of the observed grasslandification-associated effects on multifunctionality in surface soils, while microbial carbon-nitrogen fractions drove deeper soil processes. Functional gene predictions indicate differences in abundance of methanogenic genes and altered biogeochemical pathways in degraded systems, with possible implications for greenhouse gas emissions. We conclude that grasslandification is associated with compromised ecosystem resilience, driven by simplified microbial networks and reduced functional redundancy, providing observational insights essential for developing evidence-based conservation strategies for threatened alpine wetland ecosystems and similar systems elsewhere.