Effects of two common macrophytes on methane dynamics in freshwater sediments

The methane cycle in constructed wetlands without plants and with Phragmites australis (reed) and Scirpus lacustris (bulrush) was investigated. Variations in CH4 production largely determined variations in CH4 emission among the systems: rather than variations in CH4 storage and oxidation. Twofold lower CH4 production rates in the Scirpus system (5.6-13 mmol m(-2) d(-1)) relative to the control (16.7-17.6 mmol m(-2) d(-1)) were accompanied by a lower contribution of methanogenesis to organic carbon metabolism (similar to 20% for Scirpus vs. similar to 80% for control). Sedimentary iron(II) reservoirs were smaller in the Scirpus than control sediment (similar to 300 vs. similar to 485 mmol.m(-2)) and a shuttle role for iron as an intermediate between root O-2 release and carbon oxidation, attenuating the availability of substrate for methanogens, is suggested. Differences in CH4 production among the Phragmites and Scirpus systems were controlled by the interspecific variation in sediment oxidation capacities of both plant species. Comparatively, in the Phragmites sediment, dissolved iron reservoirs were larger (similar to 340 mmol.m(-2)) and methanogenesis was a more important pathway (similar to 80%). Methane transport was mainly plant mediated in the Phragmites and Scirpus systems, but ebullition dominated in the non-vegetated control systems as well as in the vegetated systems when plant biomass was low.