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Abstract

The spatial distribution of the deuterium content of precipitation has a well-established latitudinal variation that is reflected in organic molecules in plants growing at different locations. Some laboratory and field studies have already shown that the deuterium content of methane emitted from methanogens can be partially related to delta D variations of the water in the surrounding environment. Here we present a similar relation for the methane emitted from plant biomass under UV radiation. To show this relation, we determined the hydrogen isotopic composition of methane released from leaves of a range of plants grown with water of different deuterium content (delta D = -130 parts per thousand to +115 parts per thousand). The plant leaves were irradiated with UV light and the CH₄ isotopic composition was measured by continuous flow isotope ratio mass spectrometry (CF-IRMS). Furthermore, the deuterium content of bulk biomass and of the methoxyl (OCH3) groups of the biomass was measured. The D/H ratio successively decreases from bulk biomass (delta D = -106 parts per thousand to-50 parts per thousand) via methoxyl groups (SD = -310 parts per thousand to -115 parts per thousand) to the CH₄ emitted (SD = -581 parts per thousand to -196 parts per thousand). The range of isotope ratios in bulk biomass and OCH3 groups is smaller than in the water used to grow the plants. Methoxyl groups, which contain only non-exchangeable hydrogen, can be used to assess the fraction of external water that was incorporated before OCH3 groups were formed. Surprisingly, the CH₄ formed under UV irradiation has a wider isotopic range than the OCH3 groups. Although the precise production pathway cannot be fully determined, the presented experiments indicate that methoxyl groups are not the only source substrate for CH₄, but that other sources, including very depleted ones, must contribute. The main limitation to the interpretation of the data is the possible influence of exchangeable water, which could not be quantified. Future studies should include measurements of leaf water and avoid interaction between different plants via the gas phase. Despite these deficiencies, the results suggest that the deuterium content of the methane generated from plants under UV irradiation is closely linked to SD in precipitation. This dependency, which should also exist for other biogenic methane sources could be evaluated with global isotope models. (C) 2010 Elsevier Ltd. All rights reserved.