Non-Microbial Methane Formation in Plants
The results of experimental studies aimed at determining methane formation mechanisms in the aerobic plant phyllosphere and the contribution of vegetation to global methane emission are reviewed and summarized. Until recently, methane formation and emission by biogenic sources had been associated exclusively with activities of methanogenic archaea growing under anaerobic conditions of water bodies and streams, swamps, rice fields, dumps, and the gastrointestinal tract of animals and termites. However, as demonstrated by modern data, methane formation is also possible via not only the microbial route, in the aerobic plant phyllosphere. Although the mechanism of anaerobic methane formation in plants is not clearly identified, the interconnection between the observed liberation of methane by plants and UV radiation impact and other physiological stresses (temperature change and plant physical trauma) recorded on the example of numerous experimental works demonstrates that this is a general process occurring in the presence of oxygen. It is considered that during UV radiation impact and other physiological stresses on plants, chemical reactions with the formation of oxygen active species start and consequently, some amount of methane is liberated from methoxyl groups of plant pectins (and according to some data, also from plant cellulose and lignin) as part of cell dissolution process. Using very diverse approaches, it is estimated that the most probable range of total methane emission by vegetation is 20-60 Tg/yr. Herewith, the fraction of foliar methane emission related to ultraviolet irradiation of pectin does not exceed 5% of global methane emission by vegetation, ~60% of which falls on tropical latitudes.