Novel\nChemical Routes\nfor Carbon Dioxide and Methane\nProduction from Lignin Photodegradation: The Role of Environmental\nFree Radicals

Sunlight irradiation significantly\nmediates plant litter’s\ncarbon dynamics and volatile carbon release in semi-arid and arid\necosystems. In this process, carbon loss is controlled by lignin,\nbut the mechanisms of production of CO<sub>2</sub> and CH<sub>4</sub> during lignin photolysis are ambiguous. In this study, the photomineralization\nof plant litter and the lignocellulosic component collectively indicate\nthat lignin is a major source of CO<sub>2</sub> and CH<sub>4</sub> emissions. Characterization and free radical analysis reveal that\nthe production of CO<sub>2</sub> is due to the oxidation and ring-opening\nreaction of the coniferyl alcohol unit, with the subsequent decarboxylation\nof carboxylic acid as an oxidation product. This reaction involves <i>o</i>-quinone formation by the reactions between O<sub>2</sub>, superoxide radical (O<sub>2</sub><sup>·–</sup>), and\npersistent free radicals (PFRs)-bearing lignin. Of this, O<sub>2</sub><sup>·–</sup> contributes to 43.2% of the photogenerated\nCO<sub>2</sub>, as a new pathway, derived from the electron transfer\nfrom PFRs to O<sub>2</sub>. Interestingly, photoinduced demethylation\nof the dimethoxybenzene-type compounds as the photolysis products\nof lignin results in a never-before-reported CH<sub>4</sub> formation\nchemical route independent of that of O<sub>2</sub>. This mechanistic\ninsight into the role of lignin in volatile carbon production from\nthe irradiative plant litter will contribute to a deeper understanding\nof carbon balance in water-limited ecosystems.