Novel Chemical Routes for Carbon Dioxide and Methane Production from Lignin Photodegradation: The Role of Environmental Free Radicals

Sunlight irradiation significantly mediates plant litter's carbon dynamics and volatile carbon release in semi-arid and arid ecosystems. In this process, carbon loss is controlled by lignin, but the mechanisms of production of CO₂ and CH₄ during lignin photolysis are ambiguous. In this study, the photomineralization of plant litter and the lignocellulosic component collectively indicate that lignin is a major source of CO₂ and CH₄ emissions. Characterization and free radical analysis reveal that the production of CO₂ is due to the oxidation and ring-opening reaction of the coniferyl alcohol unit, with the subsequent decarboxylation of carboxylic acid as an oxidation product. This reaction involves o-quinone formation by the reactions between O₂, superoxide radical (O₂^·-), and persistent free radicals (PFRs)-bearing lignin. Of this, O₂^·- contributes to 43.2% of the photogenerated CO₂, as a new pathway, derived from the electron transfer from PFRs to O₂. Interestingly, photoinduced demethylation of the dimethoxybenzene-type compounds as the photolysis products of lignin results in a never-before-reported CH₄ formation chemical route independent of that of O₂. This mechanistic insight into the role of lignin in volatile carbon production from the irradiative plant litter will contribute to a deeper understanding of carbon balance in water-limited ecosystems.