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 CO 2 and CH 4 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 CO 2 and CH 4 emissions. Characterization and free radical analysis reveal that the production of CO 2 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 O 2 , superoxide radical (O 2 ·- ), and persistent free radicals (PFRs)-bearing lignin. Of this, O 2 ·- contributes to 43.2% of the photogenerated CO 2 , as a new pathway, derived from the electron transfer from PFRs to O 2 . Interestingly, photoinduced demethylation of the dimethoxybenzene-type compounds as the photolysis products of lignin results in a never-before-reported CH 4 formation chemical route independent of that of O 2 . 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.