James D. Happell

Relationships between CH4 emission, biomass, and CO2 exchange in a subtropical grassland

Methane flux was linearly correlated with plant biomass (r = 0.97, n = 6 and r = 0.95, n = 8) at two locations in a Florida Everglades Cladium marsh. One location, which had burned 4 months previously, exhibited a greater increase in methane flux as a function of biomass relative to sites at an unburned location. However, methane flux data from both sites fit a single regression (r = 0.94, n = 14) when plotted against net CO2 exchange suggesting that either methanogenesis in Everglades marl sediments is fueled by root exudation below ground, or that factors which enhance photosynthetic production and plant growth are also correlated with methane production and flux in this oligotrophic environment. The data presented are the first to show a direct relationship between spatial variability in plant biomass, net ecosystem production, and methane emission in a natural wetland.

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 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 peat 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 ‰, n = 5) after plants were clipped below the water surface was enriched in 13C by over 10 ‰ relative to CH4 emitted from vegetated plots (δ13C = −68.1 ± 2.5 ‰, n = 10). Methane within flood water (before clipping) at peat sites was also 13C enriched (δ13C = −57.6 ± 4.3 ‰, 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−2d−1.

Use of tritium and helium to define groundwater flow conditions in Everglades National Park

The concentrations of tritium ( 3 H) and helium isotopes ( 3 He and 4 He) were used as tracers of groundwater flow in the surficial aquifer system (SAS) beneath Everglades National Park (ENP), south Florida. From ages determined by 3 H/ 3 He dating techniques, groundwater within the upper 28 m originated within the last 30 years. Below 28 m, waters originated prior to 30 years before present with evidence of mixing at the interface. Interannual variation of the 3 H/ 3 He ages within the upper 28 m was significant throughout the 3 year investigation, corresponding with varying hydrologic conditions. In the region of Taylor Slough Bridge, younger groundwater was consistently detected below older groundwater in the Biscayne Aquifer, suggesting preferential flow to the lower part of the aquifer. An increase in He with depth in the SAS indicated that radiogenic 4 He produced in the underlying Hawthorn Group migrates into the SAS by diffusion. Higher Δ 4 He values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of 4 He in the seawater mixing zone. Groundwater salinity measurements indicated the presence of a wide (6-28 km) seawater mixing zone. Comparison of groundwater levels with surface water levels in this zone indicated the potential for brackish groundwater discharge to the overlying Everglades surface water.

Evidence for the removal of CFC-11, CFC-12, and CFC-113 at the groundwater–surface water interface in the Everglades

Abstract Poor agreement between 3 H/ 3 He ages and CFC-11 and CFC-12 ages suggests that CFCs may not be conservative tracers in the Everglades National Park. 3 H/ 3 He ages were used to calculate the expected concentration of CFC-11 and CFC-12 in groundwater from wells 2 to 73 m deep. The expected concentrations of CFCs were compared to the measured concentrations and plots of the % CFC-12 and CFC-11 remaining offered no evidence that significant CFC removal was occurring in the groundwater at depths ≥2 m, suggesting that CFC removal occurs at shallower depths. Except where CFC contamination was suspected, CFC-11, CFC-12 and CFC-113 concentrations in fresh surface water were nearly always below solubility equilibrium with the atmosphere. Measurements of CFC-11, CFC-12 and CFC-113 in pore water indicate a 50–90% decrease in concentration 5 cm below the groundwater–surface water (GW–SW) interface. In the same 5 cm interval CH 4 concentrations increased by 300–1000%. This suggested that CFCs were removed at the GW–SW interface, possibly by methane-producing bacteria. CFC derived recharge ages should therefore be viewed with caution when recharging water percolates through anoxic methanogenic sediments.

The atmospheric partial lifetime of carbon tetrachloride with respect to the global soil sink

The magnitude of the terrestrial soil sink for atmospheric carbon tetrachloride (CCl4) remains poorly constrained, with the estimated uncertainty range of CCl4 partial lifetimes between ~110 and 910 years. Field observations are sparse, and there are uncertainties in extrapolating these results to the global scale. Here we add to the published CCl4 fluxes with additional field measurements, and we employ a land cover classification scheme based on Advanced Very High Resolution Radiometer measurements that align more closely with the measurement sites to reevaluate the global CCl4 soil sink. We calculate an updated partial lifetime of CCl4 with respect to the soil sink to be 375 (288–536) years, which is 50 to 90% longer than the most recently published best estimates of the soil sink partial lifetime (195 and 245 years). This translates into a longer overall atmospheric lifetime estimate, which is more consistent with the observed atmospheric concentration trend and interhemispheric gradient.

Microbial Removal of Atmospheric Carbon Tetrachloride in Bulk Aerobic Soils

ABSTRACT Atmospheric concentrations of carbon tetrachloride (CCl4) were removed by bulk aerobic soils from tropical, subtropical, and boreal environments. Removal was observed in all tested soil types, indicating that the process was widespread. The flux measured in field chamber experiments was 0.24 ± 0.10 nmol CCl4 (m2 day)−1 (average ± standard deviation [SD]; n = 282). Removal of CCl4 and removal of methane (CH4) were compared to explore whether the two processes were linked. Removal of both gases was halted in laboratory samples that were autoclaved, dry heated, or incubated in the presence of mercuric chloride (HgCl2). In marl soils, treatment with antibiotics such as tetracycline and streptomycin caused partial inhibition of CCl4 (50%) and CH4 (76%) removal, but removal was not affected in soils treated with nystatin or myxothiazol. These data indicated that bacteria contributed to the soil removal of CCl4 and that microeukaryotes may not have played a significant role. Amendments of methanol, acetate, and succinate to soil samples enhanced CCl4 removal by 59%, 293%, and 72%, respectively. Additions of a variety of inhibitors and substrates indicated that nitrification, methanogenesis, or biological reduction of nitrate, nitrous oxide, or sulfate (e.g., occurring in possible anoxic microzones) did not play a significant role in the removal of CCl4. Methyl fluoride inhibited removal of CH4 but not CCl4, indicating that CH4 and CCl4 removals were not directly linked. Furthermore, CCl4 removal was not affected in soils amended with copper sulfate or methane, supporting the results with MeF and suggesting that the observed CCl4 removal was not significantly mediated by methanotrophs.