Physiological processes affecting methane transport by wetland vegetation – A review

Wetland plants transport oxygen to belowground tissues to survive in anoxic sediments, and simultaneously conduct methane (CH4) from the sediment to the atmosphere. Although plant-mediated transport is the main CH4 emission pathway in vegetated wetlands, the contribution of vegetated areas to total emissions in wetlands remains uncertain. To accurately quantify these emissions, understanding the physiological processes driving plant-mediated CH4 transport is crucial. This review describes the state of the art understanding of CH4 transport through trees, emergent, floating-leaved, and submerged freshwater macrophytes. Gas transport mechanisms in plants include diffusion, pressurized flow, and transpiration-driven flow. Pressurized flow in the gas-filled aerenchyma leads to higher gas transport rates than diffusion, and mostly occurs in plants standing in deeper water. Transpiration-driven flow occurs in the xylem tissue of trees, whereby dissolved CH4 is transported by sap flow. Pressurized flow and transpiration-driven flow both result in diel cycles in CH4 emission, with higher emissions during the day than at night. The total CH4 emission through a wetland plant depends on its growth stage, transport mechanisms and the balance between sediment and in-plant CH4 production and oxidation. Although plants contribute substantially to total CH4 emissions, soil carbon content, soil temperature, nutrient availability, and water depth are often stronger driving factors than plant species. Nevertheless, accurate quantification of emissions from vegetated wetlands requires standardization of measurement protocols which capture diurnal and seasonal variation in emissions. Knowledge on CH4 transport through trees and submersed and free-floating macrophytes is scarce and warrants further research.