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Abstract

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 [delta]¹³C value. Furthermore, [delta]¹³C 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 [delta]¹³C of emitted CH₄ and methoxyl groups implies that methoxyl groups are not the only source substrate of CH₄. Interestingly, [delta]D 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 [delta]D analyses may be compromised by a large reservoir of exchangeable hydrogen, but no significant [delta]D difference is found either for the methoxyl groups, which do not contain exchangeable hydrogen. The [delta]D 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.