Ma, A.; Lu, J., and Wang, T., 2012. Effects of elevation and vegetation on methane emissions from a freshwater estuarine wetland.

Methane (CH₄) emission rates from three different tidal elevations were measured in a freshwater wetland of the Yangtze River, China, estuary using a modified, static, closed-chamber technique. The intertidal zone covered by Phragmites australis is a source of atmospheric CH₄. The site at a higher elevation had lower CH₄ emissions than did a lower-elevation site (1.322 ± 1.15 mg m⁻² h⁻¹ vs. 11.66 ± 12.5 mg m⁻² h⁻¹). Methane emissions from the mudflats were negligible. Methane emissions showed remarkable seasonal variations, with the largest emissions occurring in summer and late-summer and the smallest emissions occurring in early spring. Although CH₄ emissions were related to soil temperature at depths of 5–10 cm, differences in CH₄ emissions among sites were mainly determined by the presence of P. australis. Simulatied experiments did not find significant differences in CH₄ fluxes between sites exposed to different submergence frequencies in the absence of plants, presumably because of substrate limitations. A higher tidal submergence frequency in the vegetated areas stimulated methanogenesis by enhancing vegetation photosynthesis and increasing the amount of plant litter submerged in water. The CH₄ flux rates from the clipped and unclipped sites differed significantly during the growing season. This highlights the importance of plants in the CH₄ flux, both as gas-transport conduits and through labile carbon production. The average net photosynthetic rate of P. australis at the lower elevation was higher (0.57 ± 0.2 gC m⁻² h⁻¹) than that at the higher elevation (0.43 ± 0.1 gC m⁻² h⁻¹), and more carbon was fixed and served as potential substrates for CH₄ production. The CH₄-C emissions played a minimal role in the carbon flux, which was about 0.3% for the higher-elevation site and 1.6% for the lower-elevation site. This suggests that high CH₄ emission rates in freshwater wetlands may be explained by higher plant photosynthetic carbon fixation.