The stable isotope signature of methane emitted from plant material under UV irradiation

Recent experiments have shown that dry and fresh leaves, other plant matter, as well as several structural plant components, emit methane upon irradiation with UV light. Here we present the source isotope signatures of the methane emitted from a range of dry natural plant leaves and structural compounds. UV-induced methane from organic matter is strongly depleted in both 13C and D compared to the bulk biomass. The isotopic content of plant methoxyl groups, which have been identified as important precursors of aerobic methane formation in plants, falls roughly halfway between the bulk and CH4 isotopic composition. C3 and C4/CAM plants show the well-established isotope difference in bulk 13C content. Our results show that they also emit CH4 with different δ13C value. Furthermore, δ13C of methoxyl groups in the plant material, and ester methoxyl groups only, show a similar difference between C3 and C4/CAM plants. The correlation between the δ13C of emitted CH4 and methoxyl groups implies that methoxyl groups are not the only source substrate of CH4. stingly, δD values of the emitted CH4 are also found to be different for C3 and C4 plants, although there is no significant difference in the bulk material. Bulk δD analyses may be compromised by a large reservoir of exchangeable hydrogen, but no significant δD difference is found either for the methoxyl groups, which do not contain exchangeable hydrogen. The δD difference in CH4 between C3 and C4 plants indicates that at least two different reservoirs are involved in CH4 emission. One of them is the OCH3 group, the other one must be significantly depleted, and contribute more to the emissions of C3 plants compared to C4 plants. In qualitative agreement with this hypothesis, CH4 emission rates are higher for C3 plants than for C4 plants.