Symbiotic nitrogen fixation sustains carbon sinks under warming Tibetan permafrost region

Symbiotic nitrogen fixation (SNF) by legumes is a major natural source of reactive nitrogen that supports ecosystem functioning and carbon sequestration. Warming across the Tibetan Plateau alpine permafrost region may trigger substantial shifts in vegetation composition and distribution, yet the consequences for ecosystem nitrogen inputs remain poorly understood. Here, using 1047 plant samples from 323 plots across the Tibetan Plateau and 15N natural-abundance techniques, we quantify legume and non-legume nitrogen sources and regional aboveground SNF rates. Aboveground SNF is tightly linked to legume aboveground biomass, ranges from 0 to 7.43 g N m−2 yr−1, and peaks at elevations of 3600–4000 m. At the regional scale, its spatial distribution largely coincides with recent decadal gains in aboveground biomass and plant nitrogen density. Process-based simulations show that removing SNF causes severe nitrogen limitation and shifts these ecosystems from a carbon sink to a source. Our findings highlight the important role of legumes in sustaining carbon sinks in Tibetan Plateau permafrost ecosystems. Aboveground symbiotic nitrogen fixation by legumes provides a critical nitrogen source that supports plant biomass growth and sustains carbon sequestration in warming Tibetan permafrost grasslands, based on nitrogen isotope analyses and ecosystem modeling.