Traditionally, methane (CH_4) emission from terrestrial plants is thought to originate from belowground microbial metabolism under anaerobic conditions, with subsequent transport to the atmosphere through stems. However, a recent study reported aerobic CH_4 emission from plants by an unrecognized process, a result that has since been questioned. We investigated CH_4 emissions under aerobic conditions from aboveground tissues of 44 species indigenous to the temperate Inner Mongolia steppe. Ten herbaceous hydrophytes (wetland-adapted plants) were examined, two of which-Glyceria spiculosa and Scirpus yagara-emitted CH_4 from stems but not from detached leaves. Of 34 xerophytes (arid-adapted plants) examined, 7 out of 9 shrub species emitted CH_4 from detached leaves but not stems, whereas none of 25 herbaceous xerophytes emitted CH4. The herbaceous hydrophyte, S. yagara, emitted highly ~(13)C-depleted CH_4, suggesting a microbial origin. Achillea frigida exhibited the highest CH_4 emission rates among the shrubs and continuously emitted relatively ~(13)C-enriched CH4 from detached leaves, indicating that CH_4 was derived directly from plant tissues under aerobic conditions. Because woody species are relatively rare in the Inner Mongolia steppe, aerobic, plant-derived CH_4 emission is probably negligible in this region. Our results may imply a larger role for aerobic CH_4 production in upland ecosystems dominated by woody species or in ecosystems where woody encroachment is occurring as a result of global change.
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