Gary J. Whiting

Greenhouse carbon balance of wetlands: methane emission versus carbon sequestration

Carbon fixation under wetland anaerobic soil conditions provides unique conditions for long-term storage of carbon into histosols. However, this carbon sequestration process is intimately linked to methane emission from wetlands. The potential contribution of this emitted methane to the greenhouse effect can be mitigated by the removal of atmospheric CO2 and storage into peat. The balance of CH4 and CO2 exchange can provide an index of a wetland’s greenhouse gas (carbon) contribution to the atmosphere. Here, we relate the atmospheric global warming potential of methane (GWPM) with annual methane emission/carbon dioxide exchange ratio of wetlands ranging from the boreal zone to the near-subtropics. This relationship permits one to determine the greenhouse carbon balance of wetlands by their contribution to or attenuation of the greenhouse effect via CH4 emission or CO2 sink, respectively. We report annual measurements of the relationship between methane emission and net carbon fixation in three wetland ecosystems. The ratio of methane released to annual net carbon fixed varies from 0.05 to 0.20 on a molar basis. Although these wetlands function as a sink for CO2, the 21.8-fold greater infrared absorptivity of CH4 relative to CO2(GWPM) over a relatively short time horizon (20 years) would indicate that the release of methane still contributes to the overall greenhouse effect. As GWPM decreases over longer time horizons (100 years), our analyses suggest that the subtropical and temperate wetlands attenuate global warming, and northern wetlands may be perched on the “greenhouse compensation” point. Considering a 500-year time horizon, these wetlands can be regarded as sinks for greenhouse gas warming potential, and thus attenuate the greenhouse warming of the atmosphere.

Relationship Between Ecosystem Productivity and Photosynthetically Active Radiation for Northern Peatlands

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5–50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.

Methane emission from rice: Stable isotopes, diurnal variations, and CO2 exchange

The importance of vegetation in supporting methane production and emission within flooded rice fields was demonstrated. Methane emission from Lousiana, United States, rice fields was correlated to the quantity of live aboveground biomass and the rate of CO2 exchange. The quantity of belowground methane was greater in vegetated plots relative to plots maintained free of vegetation. The diurnal maximum in the rate of methane emission was coincident with the release of the most 13C-enriched methane and a maximum in transpiration rate rather than stomatal conductance, suggesting that diurnal variations in methane emission rate are linked with transpiration, in addition to temperature. Results of isotopic measurements of belowground, lacunal, and emitted methane indicate that methane is transported from rice predominantly via molecular diffusion with a small component due to transpiration-induced bulk flow. Samples of methane collected from air-filled internal spaces within the rice culm were 13C-enriched (−53.1 ± 0.3‰) relative to emitted (−64.5 ± 1.0‰) and belowground methane (−59 ± 1.0‰) . Reproduction of these observed 13C values with a numerical model required isotopic fractionation effects associated with transport of methane into and from rice plants. The model could not conclusively confirm rhizospheric methane oxidation. However, 13C-enriched methane was observed in the floodwater overlying the flooded soil (−44.4 ± 2.2‰), consistent with the oxidation of substantial quantities of methane as it diffused across the soil-water interface.

Control of the diurnal pattern of methane emission from emergent aquatic macrophytes by gas transport mechanisms

Methane emissions from Typha latifolia L. showed a large mid-morning transient peak associated with rising light levels. This peak was also associated with a steep decline in lacunal CH4 concentrations near the stem base. This pattern contrasted sharply with emissions from Peltandra virginica (L.) Kunth that gradually rose to a peak in the mid-afternoon, corresponding to elevated air temperatures. Internal CH4 concentrations within P. virginica stems did not change significantly over the diurnal period. Stomatal conductance appeared to correlate directly with light levels in both plant types and were not associated with peak CH4 emission events in either plant. These patterns are consistent with a convective throughflow and diffusive gas ventilation systems for Typha and Peltandra, respectively. Further effects of the convective throughflow in T. latifolia were evident in the elevated CH4 concentrations measured within brown leaves as contrasted to the near ambient levels measured within live green leaves. Experimental manipulation of elevated and reduced CO2 levels in the atmosphere surrounding the plants and of light/dark periods suggested that stomatal aperture has little or no control of methane emissions from T. latifolia.

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.

Methane stable isotope distribution at a Carex dominated fen in north central Alberta

The methane stable isotope distribution was characterized at a Carex dominated fen in boreal Alberta, Canada, over three growing seasons to examine methane production, oxidation, and transport to the atmosphere; processes which are strongly tied to emergent vegetation and the influence of the rhizosphere (upper 20 cm of peat in this system]. At times when standing floodwater was present, δ13C values of emitted methane averaged −63.6 ± 2.3, −66.3 ± 1.6, and −65.4 ± 1.3‰ for the 1994, 1995, and 1996 seasons, respectively. These emissions were significantly 13C depleted relative to the belowground methane dissolved in rhizospheric pore waters, indicating that gas transport in Carex is dominated by passive diffusion. The rhizosphere was 13CH4 enriched relative to depths below the rhizosphere, consistent with the occurrence of root associated methane oxidation, preferential mobilization of 13CH4, and a relatively greater role of acetate fermentation type methane production. Dual isotope tracers, δ13C and δD, help qualify the role of each of these processes and aid in describing the distribution of production pathways, CO2 reduction, and acetate fermentation. Inverse trends in δ13C-CH4 and δD-CH4 depth profiles are consistent with an interpretation suggesting an evolution toward methane production by CO2 reduction with increasing depth. A shift in production mechanisms appears to be the dominate process affecting the stable isotope distribution below 10 cm in the peat column, while oxidation and transport isotope effects are dominant above 10 cm. To test several hypotheses regarding the effects of transport, oxidation, and production on methane isotope distributions, we also present measurements from sites fertilized and sites devegetated (continually clipped) over the 3 year period. Removal of vegetation quickly halted rhizospheric methane oxidation and gas transport while gradually increasing the relative role of CO2 reduction in net methane production as labile substrate was used up. The fertilizer treatment increased above ground biomass and primary productivity but had little effect on the stable isotope distribution. A mass balance calculation indicates that methane emissions are attenuated 0–34% by methane oxidation in the rhizosphere. Results showed little seasonal variability other than during a period when floodwater levels dropped below the peat surface resulting in the13CH4 enrichment of methane emissions.

Evaluation of methane oxidation in the rhizosphere of a Carex dominated fen in north central Alberta, Canada

Rhizospheric methane oxidation was evaluated at a Carex (spp.) dominated fen in Alberta, Canada overthree growing seasons. Aerobic incubations of bulkpeat and live roots in the laboratory show a clearassociation between active methane oxidizing bacteriaand the rhizosphere. Aerobic incubations also show anoxidation potential that far exceeds methaneproduction potential measured in the laboratory. Quantitative estimates of how this oxidation potentialis expressed in situ depend strongly on which of twocommon approaches are used. (1) Subtracting in situmethane emission rates from methane production ratesmeasured in the laboratory with anaerobic incubationssuggest that methane oxidation may attenuate emissionsby 58 to 92%. (2) Applying the inhibitor methylfluoride (CH3F) to whole plants in situ suggestmethane oxidation attenuates emissions by less than20% seasonally. The production minus emissiontechnique likely overestimates methane oxidationbecause methane production measured via anaerobicincubations in the laboratory are probablyoverestimates. Oxidation percentages measured byCH3F were greatest early in the growing seasonwhen emission rates were low and fell to almostnondetectable levels as emission rates peaked in latesummer. Estimates provided by the CH3F techniquewere generally in better agreement with estimates ofoxidation based on a stable isotope mass balance(0–34%) determined in a companion study (Popp et al. 1999).

Biosphere/atmosphere CO2 exchange in tundra ecosystems: Community characteristics and relationships with multispectral surface reflectance

The spatial and temporal patterns of many of the factors controlling CO2 exchange are related to characteristics of the vegetated surface which can potentially be monitored using multispectral remote sensors. Realization of this potential depends, in part, on an improved understanding of ecosystem processes and their relationship to variables which are accessible to remote sensing techniques. We examined these relationships using portable, climate-controlled, instrumented enclosures to measure CO2 exchange rates in selected tundra sites near Bethel, Alaska. Rates were related to vegetation community type and climatic variables. Exchange rates in enclosures were compared to exchange measurements obtained by eddy correlation on a 12-m micrometeorological tower. For an average light input of 37 einsteins/day during 20 midsummer days, the empirically modeled exchange rate for a representative area of vegetated tundra was 1.2 ±1.1 (95% confidence interval) g CO2 m−2 d−1. This was comparable to a tower measured exchange over the same time period of 1.1 ±1.1 (95% confidence interval) g CO2 m−2 d−1. Net exchange in response to varying light levels was compared for two major community types, wet meadow and dry upland tundra, and to the net exchange measured by the micrometeorological tower technique. Portable radiometers were used to measure the multispectral reflectance properties of the sites. These properties were then related to exchange rates with the goal of providing a quantitative foundation for the use of satellite remote sensing to monitor biosphere/atmosphere CO2 exchange in the tundra biome. The Normalized Difference Vegetation Index (NDVI) and the near-infrared/red reflectance ratio (SR) computed from surface reflectance were strongly correlated with net CO2 exchange for both upland and wet meadow vegetation. However, the form of the relationship was distinct from measured correlations in other ecosystems, suggesting that global surveys may require adjustment for geographical differences in exchange processes.

CO2 exchange in the Hudson Bay lowlands: Community characteristics and multispectral reflectance properties

Net ecosystem CO2 exchange was measured during the 1990 growing season (June to August) along a transect starting 10 km inland from James Bay and extending 100 km interior to Kinosheo Lake, Ontario. Sites were chosen in three distinct areas: a coastal fen, an interior fen, and a bog. For the most productive sites in the bog, net daily uptake rates reached a maximum of 2.5 g C-CO2 m−2 d−1 with an area-weighted exchange of 0.3 g C-CO2 m−2 d−1 near midsummer. This site was estimated to be a net carbon source of 9 g C-CO2 m−2 to the atmosphere over a 153-day growing season. The interior fen was less productive on a daily basis with a net maximum uptake of 0.5 g C-CO2 m−2 d−1 and with corresponding area-weighted uptake of 0.1 g C-CO2 m−2 d−1 during midsummer. Early and late season release of carbon to the atmosphere resulted in a net loss of 21 g C-CO2 m−2 over the growing season from this site. The coastal fen was the most productive site with uptake rates peaking near 1.7 g C-CO2 m−2 d−1 which corresponded to an area-weighted uptake of 0.8 g C-CO2 m−2 d−1 during midsummer and an estimated net uptake of 6g C-CO2 m−2 for the growing season. Associated with net CO2 exchange measurements, multispectral reflectance properties of the sites were measured over the growing season using portable radiometers. These properties were related to exchange rates with the goal of examining the potential for satellite remote sensing to monitor biosphere/atmosphere CO2 exchange in this biome. The normalized difference vegetation index (NDVI) computed from surface reflectance was correlated with net CO2 exchange for all sites with the exception of areas with large proportions of Sphagnum moss cover. These mosses have greater near-infrared reflectance than typical surrounding vegetation and may require special adjustment for regional exchange/remote sensing applications.

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.