Wetlands: A potentially significant source of atmospheric methyl bromide and methyl chloride

Tropospheric methyl bromide (CH3Br) and methyl chloride (CH3Cl) are significant sources of ozone (O3) destroying halogens to the stratosphere. Their O3 depletion potential (ODP) can be determined from atmospheric lifetimes and therefore their atmospheric budgets, both of which are out of balance with known sink terms larger than identified sources. We have discovered a new source of CH3Br and CH3Cl emissions to the atmosphere at two wetland sites in the Northeastern United States. We have reason to believe that these compounds are biologically produced in situ. Our measurements indicate that the global annual flux of CH3Br and CH3Cl from wetlands could be as high as 4.6 Gg yr−1 Of CH3Br and 48 Gg yr−1 of CH3Cl. These are preliminary estimates based on measurements made during the end of the 1998 growing season, a time period of decreased emissions of other trace gases such as methane (CH4)

Controls on the seasonal exchange of CH3Br in temperate peatlands

Measurements of CH3Br exchange at two New Hampshire peatlands (Sallie\u27s Fen and Angie\u27s Bog) indicate that net flux from these ecosystems is the sum of competing production and consumption processes. Net CH3Br fluxes were highly variable and ranged from net emission to net uptake between locations within a single peatland. At Sallie\u27s Fen, net CH3Br flux exhibited positive correlations with peat temperature and air temperature during all seasons sampled, but these relationships were not observed at Angie\u27s Bog where flux varied according to microtopography. The major CH3Br production process at Sallie\u27s Fen appeared dependent on aerobic conditions within the peat, while CH3Br production at Angie\u27s Bog was favored by anaerobic conditions. There was evidence of aerobic microbial consumption of CH3Br within the peat at both sites. In a vegetation removal experiment conducted at Sallie\u27s Fen with dynamic chambers, all collars exhibited net consumption of CH3Br. Net CH3Br flux had a negative correlation with surface temperature and a positive correlation with water level in collars with all vegetation clipped consistent with aerobic microbial consumption. Vegetated collars showed positive correlations between net CH3Br flux and air temperature. A positive correlation between net CH3Br flux and surface temperature was also observed in collars in which all vegetation except Sphagnum spp. were clipped. These correlations are consistent with seasonal relationships observed in 1998, 1999, and 2000 and suggest that plants and/or fungi are possible sources of CH3Br in peatlands. Estimates of production and consumption made on two occasions at Sallie\u27s Fen suggest that peatlands have lower rates of CH3Br consumption compared to upland ecosystems, but a close balance between production and consumption rates may allow these wetlands to act as either a net source or sink for this gas

Timescale dependence of environmental and plant‐mediated controls on CH4 flux in a temperate fen

This study examined daily, seasonal, and interannual variations in CH4 emissions at a temperate peatland over a 5‐year period. We measured net ecosystem CO2 exchange (NEE), CH4 flux, water table depth, peat temperature, and meteorological parameters weekly from the summers (1 May to 31 August) of 2000 through 2004 at Sallie\u27s Fen in southeastern New Hampshire, United States. Significant interannual differences, driven by high variability of large individual CH4 fluxes (ranging from 8.7 to 3833.1 mg CH4 m−2 d−1) occurring in the late summer, corresponded with a decline in water table level and an increase in air and peat temperature. Monthly timescale yielded the strongest correlations between CH4 fluxes and peat and air temperature (r2 = 0.78 and 0.74, respectively) and water table depth (WTD) (r2 = 0.53). Compared to daily and seasonal timescales, the monthly timescale was the best timescale to predict CH4 fluxes using a stepwise multiple regression (r2 = 0.81). Species composition affected relationships between CH4 fluxes and measures of plant productivity, with sedge collars showing the strongest relationships between CH4 flux, water table, and temperature. Air temperature was the only variable that was strongly correlated with CH4 flux at all timescales, while WTD had either a positive or negative correlation depending on timescale and vegetation type. The timescale dependence of controls on CH4 fluxes has important implications for modeling

An estimate of the uptake of atmospheric methyl bromide by agricultural soils

Published estimates of removal of atmospheric methyl bromide (CH3Br) by agricultural soils are 2.7 Gg yr−1 (Gg = 109 g) [Shorter et al., 1995] and 65.8 Gg yr−1 [Serça et al., 1998]. The Serça et al. estimate, if correct, would suggest that the current value for total removal of atmospheric CH3Br by all sinks of 206 Gg yr−1 (based on Shorter et al., 1995) would be 30% too low. We have calculated a new rate of global agricultural soil uptake of atmospheric CH3Br from a larger sampling of cultivated soils collected from 40 sites located in the United States, Costa Rica, and Germany. First order reaction rates were measured during static laboratory incubations. These data were combined with uptake measurements we reported earlier based on field and laboratory experiments [Shorter et al. 1995]. Tropical (10.2°–10.4°N) and northern (45°–61°N) soils averaged lower reaction rate constants than temperate soils probably due to differing physical and chemical characteristics as well as microbial populations. Our revised global estimate for the uptake of ambient CH3Br by cultivated soils is 7.47±0.63 Gg yr−1, almost three times the value that we reported in 1995

Production of methyl bromide in a temperate forest soil

Field enclosure measurements of a temperate forest soil show net uptake of ambient methyl bromide (CH3Br), an important trace gas in both tropospheric and stratospheric ozone cycling. The net flux for 1999 was estimated to be −168 ± 72 μg CH3Br m−2 (negative indicates loss from the atmosphere). Individual enclosure flux measurements ranged from −4.0 to +3.3 μg CH3Br m−2 d−1. Soil consumption of CH3Br was estimated from laboratory soil incubations. Production of CH3Br was calculated as the difference between net flux and predicted consumption. Fungi could be responsible for the production of CH3Br in this temperate forest soil

Emissions of sulfur gases from marine and freshwater wetlands of the Florida Everglades: Rates and extrapolation using remote sensing

Rates of emissions of the biogenic sulfur (S) gases carbonyl sulfide (COS), methyl mercaptan (MSH), dimethyl sulfide (DMS), and carbon disulfide (CS2) were measured in a variety of marine and freshwater wetland habitats in the Florida Everglades during a short duration period in October using dynamic chambers, cryotrapping techniques, and gas chromatography. The most rapid emissions of greater than 500 nmol/m(sup -2)h(sup -1) occurred in red mangrove-dominated sites that were adjacent to open seawater and contained numerous crab burrows. Poorly drained red mangrove sites exhibited lower fluxes of approximately 60 nmol/m(sup -2)h(sup -1) which were similar to fluxes from the black mangrove areas which dominated the marine-influenced wetland sites in the Everglades. DMS was the dominant organo-S gas emitted especially in the freshwater areas. Spectral data from a scene from the Landsat thematic mapper were used to map habitats in the Everglades. Six vegetation categories were delineated using geographical information system software and S gas emission were extrapolated for the entire Everglades National Park. The black mangrove-dominated areas accounted for the largest portion of S gas emissions to the area. The large area extent of the saw grass communities (42 percent) accounted for approximately 24 percent of the total S emissions

Methane flux from the Central Amazonian Floodplain

A total of 186 methane measurements from the three primary Amazon floodplain environments of open water lakes, flood forests, and floating grass mats were made over the period 18 July through 2 September 1985. These data indicate that emissions were lowest over open water lakes. Flux from flooded forests and grass mats was significantly higher. At least three transport processes contribute to tropospheric emissions: ebullition from sediments, diffusion along the concentration gradient from sediment to overlaying water to air, and transport through the roots and stems of aquatic plants. Measurements indicate that the first two of these processes are most significant. It was estimated that on the average bubbling makes up 49% of the flux from open water, 54% of that from flooded forests, and 64% of that from floating mats. If the measurements were applied to the entire Amazonian floodplain, it is calculated that the region could supply up to 12% of the estimated global natural sources of methane

Soil–Atmosphere Exchange of Nitrous Oxide, Nitric Oxide, Methane, and Carbon Dioxide in Logged and Undisturbed Forest in the Tapajos National Forest, Brazil

Selective logging is an extensive land use in the Brazilian Amazon region. The soil–atmosphere fluxes of nitrous oxide (N2O), nitric oxide (NO), methane (CH4), and carbon dioxide (CO2) are studied on two soil types (clay Oxisol and sandy loam Ultisol) over two years (2000–01) in both undisturbed forest and forest recently logged using reduced impact forest management in the Tapajos National Forest, near Santarem, Para, Brazil. In undisturbed forest, annual soil–atmosphere fluxes of N2O (mean ± standard error) were 7.9 ± 0.7 and 7.0 ± 0.6 ng N cm−2 h−1 for the Oxisol and 1.7 ± 0.1 and 1.6 ± 0.3 ng N cm−2 h−1 for the Ultisol for 2000 and 2001, respectively. The annual fluxes of NO from undisturbed forest soil in 2001 were 9.0 ± 2.8 ng N cm−2 h−1 for the Oxisol and 8.8 ± 5.0 ng N cm−2 h−1 for the Ultisol. Consumption of CH4 from the atmosphere dominated over production on undisturbed forest soils. Fluxes averaged −0.3 ± 0.2 and −0.1 ± 0.9 mg CH4 m−2 day−1 on the Oxisol and −1.0 ± 0.2 and −0.9 ± 0.3 mg CH4 m−2 day−1 on the Ultisol for years 2000 and 2001. For CO2 in 2001, the annual fluxes averaged 3.6 ± 0.4 μmol m−2 s−1 on the Oxisol and 4.9 ± 1.1 μmol m−2 s−1 on the Ultisol. We measured fluxes over one year each from two recently logged forests on the Oxisol in 2000 and on the Ultisol in 2001. Sampling in logged areas was stratified from greatest to least ground disturbance covering log decks, skid trails, tree-fall gaps, and forest matrix. Areas of strong soil compaction, especially the skid trails and logging decks, were prone to significantly greater emissions of N2O, NO, and especially CH4. In the case of CH4, estimated annual emissions from decks reached extremely high rates of 531 ± 419 and 98 ± 41 mg CH4 m−2 day−1, for Oxisol and Ultisol sites, respectively, comparable to wetland emissions in the region. We calculated excess fluxes from logged areas by subtraction of a background forest matrix or undisturbed forest flux and adjusted these fluxes for the proportional area of ground disturbance. Our calculations suggest that selective logging increases emissions of N2O and NO from 30% to 350% depending upon conditions. While undisturbed forest was a CH4 sink, logged forest tended to emit methane at moderate rates. Soil–atmosphere CO2 fluxes were only slightly affected by logging. The regional effects of logging cannot be simply extrapolated based upon one site. We studied sites where reduced impact harvest management was used while in typical conventional logging ground damage is twice as great. Even so, our results indicate that for N2O, NO, and CH4, logging disturbance may be as important for regional budgets of these gases as other extensive land-use changes in the Amazon such as the conversion of forest to cattle pasture

Experimentally induced root mortality increased nitrous oxide emission from tropical forest soils

We conducted an experiment on sand and clay tropical forest soils to test the short‐term effect of root mortality on the soil‐atmosphere flux of nitrous oxide, nitric oxide, methane, and carbon dioxide. We induced root mortality by isolating blocks of land to 1 m using trenching and root exclusion screening. Gas fluxes were measured weekly for ten weeks following the trenching treatment. For nitrous oxide there was a highly significant increase in soil‐atmosphere flux over the ten weeks following treatment for trenched plots compared to control plots. N2O flux averaged 37.5 and 18.5 ng N cm−2 h−1 from clay trenched and control plots and 4.7 and 1.5 ng N cm−2 h−1 from sand trenched and control plots. In contrast, there was no effect for soil‐atmosphere flux of nitric oxide, carbon dioxide, or methane

Fine root dynamics and trace gas fluxes in two lowland tropical forest soils

Fine root dynamics have the potential to contribute significantly to ecosystem-scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 ( 35) and 153 ( 27) gm 2 yr 1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k5 0.96 year 1) than in the sandy loam soil (k5 0.61 year 1),leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N2O) emissions were significantly greater in the clay soil (13 1ngNcm 2 h 1) than in the sandy loam (1.4 0.2 ngNcm 2 h 1). Root mortality and decay following trenching doubled rates of N2O emissions in the clay and tripled them in sandy loam over a 1-year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 gCm 2 yr 1, accounting for 24% to 35% of the total soil CO2 efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521 206 gCm2 yr 1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land-use change can contribute significantly to increased rates of nitrogen trace gas emissions