Permafrost ecosystem stores a large amount of organic carbon (C), which has the potential to become a major positive feedback to climate warming. Previous studies indicated that warming accelerated soil C emissions without changes in the total C stock, which is likely due to the increased plant C input. However, it remains unclear whether these altered C input and output will change soil organic C composition. In this study, we conducted a manipulative warming experiment in a Tibetan permafrost ecosystem to investigate the responses of plant growth, soil respiration and its components to experimental warming. Further, we employed molecular-level methods to assess the warming effects on soil organic C composition.
Four-year experimental warming significantly increased plant root biomass as well as soil respiration and its components. The accelerated soil respiration was mainly resulted from the increase in heterotrophic respiration. Warming altered soil organic C composition. The 13C nuclear magnetic resonance (NMR) analysis showed that elevated temperature caused the accumulation of recalcitrant C fraction (Alkyl carbon). Both 13C NMR and biomarker analyses indicated that experimental warming significantly decreased the lignin-derived phenols of soil organic C. The rapid decomposition of lignin under warming was resulted from the increased soil phenol oxidase activity. We suggested that this permafrost ecosystem will exert a positive feedback to climate warming due to the increase in the proportional contributions of soil heterotrophic respiration. These findings suggested that climate warming could significantly influence soil organic C composition in the Tibetan permafrost ecosystem. The warming effect on soil respiration may shift from heterotrophic to autotrophic respiration at long-term scale due to the accumulation of recalcitrant C fraction.