Nitrogen Enrichment Reshapes Contrasting Microbial Networks in Northern Tibetan Alpine Meadow vs. Steppe

Increased Nitrogen (N) input exerts significant impact on the functional integrity of terrestrial ecosystems, with alpine grasslands being particularly susceptible. Soil microbes are intricately intertwined with nearly all facets of essential biogeochemical cycle, underscoring their pivotal role in ecosystem processes. To elucidate how N enrichment modulates soil microbes and their diversity, 11-year N addition experiments were conducted in a semi-humid alpine meadow (AM) and an arid alpine steppe (AS) on the Northern Tibetan Plateau. We measured soil properties, aboveground net primary productivity (ANPP), plant diversity, microbial composition and diversity, as well as microbial co-occurrence networks. The results revealed that N additions profoundly reshaped microbial co-occurrence in alpine grasslands, albeit via divergent mechanisms in different ecosystems. In AM, N enrichment destabilized microbial networks mainly through reduced bacterial diversity linked to plant diversity loss. Conversely, in the harsher AS, N addition fostered closer microbial interactions, forming a more stable co-occurrence network despite lower plant richness, predominantly attributed to increased soil nutrient availability. Our results highlight the significance of co-occurrence networks as a key component of microbial biodiversity and emphasize the imperative of deciphering microbial interaction mechanisms to unravel soil functional dynamics under global nitrogen enrichment.