Asymmetric response of nitrous oxide emissions to active layer thawing in permafrost

Permafrost stores large amounts of organic matter. As warming-induced thawing, this organic matter becomes accessible for microbial decomposition, potentially leading to substantial nitrous oxide (N₂O) emissions. However, previous studies have primarily focused on rapid permafrost collapse and its effects on N₂O fluxes, the impact of gradual thawing of active layer remains unclear. Here, we conducted a whole-ecosystem warming experiment on the Tibetan permafrost region to simulate a 2°C increase in ecosystem temperature accompanied by increasing active layer. In-situ monitoring of N₂O fluxes was combined with ¹⁵N site preference (SP) and δ¹⁸O isotopomers of N₂O, microbial high-throughput sequencing, and meta transcriptomics to elucidate the change in N₂O emissions upon the increasing thawing of active layer and its underlying mechanisms. Our results reveal an asymmetric response of N₂O fluxes to thawing of active layer throughout the growing season. In the early growing season, thawing increased N₂O fluxes by 261 %, with significant changes in δ¹⁸O and SP, as well as change the soil organic matter, microbial diversity, and activity. In contrast, no significant effects were observed in the later season. These findings suggest that, during early-season, thawing accelerates nutrient release, alleviating nitrogen competition and promoting microbial growth, which enhances nitrification-driven N₂O emissions. During later-season, plant nutrient depletion intensifies competition, suppressing microbial activity and N₂O fluxes. This study is the first in-situ to report N₂O emissions in response to gradual active layer thawing, providing important evidence for understanding ecosystem responses to climate change.