Effects of two common macrophytes on methane dynamics in freshwater sediments

The methane cycle in constructed wetlands without plants and withPhragmites australis (reed) and Scirpus lacustris (bulrush) wasinvestigated. Variations in CH4production largely determined variations in CH4 emission among the systems, rather than variations inCH4 storage and oxidation. Twofoldlower CH4 production rates in theScirpus system (5.6–13 mmol m-2 d-1) relative to the control (16.7–17.6 mmolm-2 d-1) were accompanied by a lower contribution ofmethanogenesis to organic carbon metabolism (∼20% for Scirpus vs.∼80% for control). Sedimentary iron(II) reservoirs were smallerin the Scirpus than control sediment (∼300 vs. ∼485 mmol.m-2) and a shuttle role for iron asan intermediate between root O2release and carbon oxidation, attenuating the availability of substrate formethanogens, is suggested. Differences in CH4 production among the Phragmites and Scirpus systemswere controlled by the interspecific variation in sediment oxidationcapacities of both plant species. Comparatively, in the Phragmites sediment,dissolved iron reservoirs were larger (∼340 mmol.m-2) and methanogenesis was a more importantpathway (∼80%). Methane transport was mainly plant mediated inthe Phragmites and Scirpus systems, but ebullition dominated in thenon-vegetated control systems as well as in the vegetated systems when plantbiomass was low.