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 stand ard deviation throughout) times greater than the flux from the flood w ater. 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 tran sported through the plant mainly via passive diffusion and/or effusion as opposed to active pressurized ventilation. Rhizospheric CH4 oxidat ion did not cause C-13-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 a nd inactive in marl soils (King et al., 1990; Gerard, 1992), however a comparison of the stable isotopic composition of emitted and sediment ary CH4 from Cladium marshes within marl and peat soils provided no ev idence that rhizospheric CH4 oxidizing bacteria were consuming signifi cant quantities of CH4 in situ within peat soils. Either CH4 oxidation in the rhizosphere was insignificant due to O2 limitation or it occur red quantitatively in discrete zones within the sediment, thereby impa rting no isotopic signal to sedimentary CH4. Linear relationships betw een CH4 flux and live aboveground Cladium biomass in marl and peat soi ls were identical and offered no evidence for rhizospheric CH4 oxidati on in peat soils. In Contrast core incubation experiments indicated th at CH4 oxidizing bacteria at the sediment-water interface in peat soil s intercepted and oxidized from 41 to 93 % (avg = 71 +/- 20 %, n = 9) of the CH4 diffusing from the sediments toward the overlying flood wat er. Furthermore, we were able to detect sediment-water interface oxida tion with stable isotopes as CH4 emitted from the flood water (deltaC- 13 = 57.3 +/- 3.6 parts per thousand, n = 5) after plants were clipped below the water surface was enriched in C-13 by over 10 parts per tho usand relative to CH4 emitted from vegetated plots (deltaC-13 = -68.1 +/- 2.5 parts per thousand n = 10). Methane within flood water (before clipping) at peat sites was also C-13 enriched (deltaC-13 = -57.6 +/- 4.3 parts per thousand, 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 CH4 at a rate of 55 +/- 4 1 mumol m-2d-1.