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 maintains ecosystem functioning and carbon sequestration. The Tibetan Plateau (TP), Earth’s largest and highest alpine region, has warmed at nearly twice the global average, triggering substantial shifts in vegetation composition and distribution. However, the consequences of these changes for ecosystem nitrogen inputs remain poorly understood. Using 1,047 plant samples from 323 plots across the TP and 15N natural-abundance techniques, we quantified legume and non-legume nitrogen sources and derived regional SNF rates. We found permafrost thawing is more conducive to the growth of legumes. Site-level SNF was tightly linked to legume aboveground biomass, with rates ranging from 0 to 7.43 g N m-2 yr-1 and peaking at elevations of 3,600–4,000 m. At the regional scale, the SNF distribution largely reflecting recent decadal gains in aboveground biomass and plant N density. Process-based simulations show that removing SNF would eliminate increases in net primary production and shift Tibetan grasslands from a carbon sink to a carbon source. Our findings highlight legume SNF as a previously underappreciated mechanism buffering nitrogen limitation and sustaining carbon uptake in rapidly warming alpine permafrost ecosystems.