Stable isotopes as tracers of methane dynamics in Everglades marshes with and without active populations of methane oxidizing bacteria

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 CH 4 emission and its stable carbon isotopic composition suggests that CH 4 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 CH 4 oxidation did not cause 13 C‐enriched CH 4 to be emitted to the atmosphere by Cladium jamaicense . Previous workers have shown that Everglades soil types differ in that CH 4 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 CH 4 from Cladium marshes within marl and peat soils provided no evidence that rhizospheric CH 4 oxidizing bacteria were consuming significant quantities of CH 4 in situ within peat soils. Either CH 4 oxidation in the rhizosphere was insignificant due to O 2 limitation or it occurred quantitatively in discrete zones within the sediment, thereby imparting no isotopic signal to sedimentary CH 4 . Linear relationships between CH 4 flux and live aboveground Cladium biomass in marl and peat soils were identical and offered no evidence for rhizospheric CH 4 oxidation in peat soils. In contrast core incubation experiments indicated that CH 4 oxidizing bacteria at the sediment‐water interface in peat soils intercepted and oxidized from 41 to 93 % (avg = 71 ± 20 %, n = 9) of the CH 4 diffusing from the sediments toward the overlying flood water. Furthermore, we were able to detect sediment‐water interface oxidation with stable isotopes as CH 4 emitted from the flood water (δ 13 C = 57.3 ± 3.6 ‰, n = 5) after plants were clipped below the water surface was enriched in 13 C by over 10 ‰ relative to CH 4 emitted from vegetated plots (δ 13 C = −68.1 ± 2.5 ‰, n = 10). Methane within flood water (before clipping) at peat sites was also 13 C enriched (δ 13 C = −57.6 ± 4.3 ‰, n = 7). Lowering of the water table below the sediment surface caused an Everglades sawgrass marsh to shift from CH 4 emission to the consumption of atmospheric CH 4 at a rate of 55 ± 41 μmol m −2 d −1 .