Mechanisms of methane transport through Populus trichocarpa

Other literature type English OPEN
Kutschera, Ellynne ; Khalil, Aslam ; Rice, Andrew ; Rosenstiel, Todd (2016)

Although the dynamics of methane (CH<sub>4</sub>) emission from croplands and wetlands have been fairly well investigated, the contribution of trees to global CH<sub>4</sub> emission and the mechanisms of tree transport are relatively unknown. CH<sub>4</sub> emissions from the common wetland tree species <i>Populus trichocarpa</i> (black cottonwood) native to the Pacific Northwest were measured under hydroponic conditions in order to separate plant transport mechanisms from the influence of soil processes. Roots were exposed to CH<sub>4</sub> enriched water and canopy emissions of CH<sub>4</sub> were measured. The average flux for 34 trials (at temperatures ranging from 17 to 25 &deg;C) was 2.8 ± 2.2 <i>μ</i>g CH<sub>4</sub> min<sup>&minus;1</sup> (whole canopy). Flux increased with temperature. Compared to the isotopic composition of root water CH<sub>4</sub>, <i>δ</i><sup>13</sup>C values were depleted for canopy CH<sub>4</sub> where the warmest temperatures (24.4&ndash;28.7 &deg;C) resulted in an epsilon of 2.8 ± 4.7 ‰; midrange temperatures (20.4&ndash;22.1 &deg;C) produced an epsilon of 7.5 ± 3.1 ‰; and the coolest temperatures (16.0&ndash;19.1 &deg;C) produced an epsilon of 10.2 ± 3.2 ‰. From these results it is concluded that there are likely multiple transport processes at work in CH<sub>4</sub> transport through trees and the dominance of these processes changes with temperature. The transport mechanisms that dominate at low temperature and low flux result in a larger fractionation, while the transport mechanisms that prevail at high temperature and high flux produce a small fractionation. Further work would investigate what combination of mechanisms are specifically engaged in transport for a given fractionation of emitted CH<sub>4</sub>.
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