Methane flux from Cladium jamaicense varied from 0.2 to 15 mmol m-2 d-1 and was 1.4 to 26 (avg=5.64¿8.57, n=13, error is ¿1 standard deviation throughout) times greater than the flux from the flood water. The lack of diurnal variations in both the rate of CH4 emission and its stable carbon isotopic composition suggests that CH4 flux from Cladium was independent of stomatal aperture and that gases were transported through the plant mainly via passive diffusion and/or effusion as opposed to active pressurized ventilation. Rhizospheric CH4 oxidation did not cause 13C-enriched CH4 to be emitted to the atmosphere by Cladium jamaicense. Previous workers have shown that Everglades soil types differ in that CH4 oxidizing bacteria are active in peat soils and inactive in marl soils (King et al., 1990; Gerard, 1992), however a comparison of the stable isotopic composition of emitted and sedimentary CH4 from Cladium marshes within marl and peak soils provided no evidence that rhizospheric CH4 oxidizing bacteria were consuming significant quantities of CH4 in situ within peat soils. Either CH4 oxidation in the rhizosphere was insignificant due to O2 limitation or it occurred quantitatively in discrete zones within the sediment, thereby imparting no isotopic signal to sedimentary CH4. Linear relationships between CH4 flux and live aboveground Cladium biomass in marl and peat soils were identical and offered no evidence for rhizospheric CH4 oxidation in peat soils.

In contrast core incubation experiments indicated that CH4 oxidizing bacteria at the sediment-water interface in peat soils intercepted and oxidized from 41 to 93% (avg=71¿20%, n=9) of the CH4 diffusing from the sediments toward the overlying flood water. Furthermore, we were able to detect sediment-water interface oxidation with stable isotopes as CH4 emitted from the flood water (Δ13C=57.3¿3.6¿/oo, n=5) after plants were clipped below the water surface was enriched in 13C by over 10¿/oo relative to CH4 emitted from vegetated plots (Δ13C=-68.1¿2.5¿/oo, n=10). Methane within flood water (before clipping) at peat sites was also 13C enriched (Δ13C=-57.6¿4.3¿/oo, n=7). Lowering of the water table below the sediment surface caused an Everglades sawgrass marsh to shift from CH4 emission to the consumption of atmospheric CH4 at a rate of 55¿41 μmol m-2 d-1. ¿ American Geophysical Union 1993