Legacy Carbon Awakens: Permafrost and Grassland Responses to Himalayan Warming

Climate change threatens the Himalayas and the billions of people dependent on its resources and water. Warming temperatures lead to melting glaciers, extended growing seasons, and the degradation of permafrost and peatlands, releasing significant amounts of carbon stored for millennia. This process alters ecosystems and triggers cascading effects on soil and vegetation. How permafrost thaw alters carbon cycling within different landscapes is an open question in many ecosystems. Here, we investigate soil organic carbon dynamics in a permafrost and two wetland sites located at different elevation in the western Himalayas also characterized by cold arid climate, glacial and riverine resources, and geothermal activity in one wetalnd. Gas chambers were deployed to quantify CO₂ and CH₄ fluxes and revealed that the permafrost (Tsoltak) and wetland sites (Ganglass and Puga) are substantial sources of CH₄ during the post monsoon season in 2023. While Ganglass and Puga (water–logged sites at lower elevations) act as CO₂ sinks, Tsoltak, a more arid site at higher elevation, predominantly exhibits CO₂ emissions indicating different microbial decomposition. High methane fluxes observed at wetter locations exhibited by relatively lower stable carbon isotope ratio (δ¹³C) indicating predominance of hydrogenotrophic methanogenesis. In contrast, relatively lower CH₄ fluxes with enriched δ¹³C–CH₄ signature at Tsoltak point towards acetoclastic methanogenesis coupled with limited CH₄ oxidation. Upscaling the median permafrost carbon flux measurements from our study sites to the estimated permafrost area in the entire western Himalaya suggests a potential annual CO₂–equivalent carbon emission of up to 1.2 Tg (1 Tg = 10¹² g). The bulk soil organic matter analyzed near each chamber revealed 8.5–9.5 kg C m^-2 in permafrost soil, nearly three times the amount observed in marshy grassland. Organic matter source proxies, including bulk soil δ¹³C and biomarker (n–alkanes) characteristics such as average chain length, carbon preference index, odd–over–even preference index, and n–alkane ratio, exhibited consistent signatures across the sites. This indicates similar organic matter sources, primarily C3–type grasses, macrophytes, aquatic plants and possibly microbes. The subsurface soil-respired δ¹³C–CO₂ values were higher compared to bulk organic matter but significantly lower than the local ambient air. The ¹⁴C–CO₂ ages indicated a mixture of modern and ancient carbon sources, suggesting the release of legacy carbon from the permafrost. Our findings offer initial insights into the organic carbon cycling in degrading Himalayan permafrost and peatlands under increasing stress of global warming. This will enhance understanding and predictions of soil carbon dynamics in the warmer and wetter Himalayas projected for the late 21st century.