Potential Sulphur Gas Emissions from a Tropical Rainforest and a Southern Appalachian Deciduous Forest
Potential emission rates of reduced sulphur gases were estimated for a tropical rainforest and a southern Appalachian deciduous forest. Potential emissions were sampled by using cuvettes placed on the forest floor, by incubating samples of leaf litter and soil in closed containers, by incubating living plant material in closed containers, and by pumping air from around plant canopies enclosed in transparent Tedlar bags. A gas Chromatograph fitted with a sulphur specific detector identified and quantified sulphur gases from cuvette sampling and incubations. Quantification of potential H2S emissions from plant canopies was attempted by Zn+Na acetate trapping followed by colorimetry. Sulphur emissions were not detected with cuvette sampling. Potential sulphur emission rates (± standard deviation) from the litter of the tropical rainforest at La Selva, Costa Rica estimated iy ipcubations were 5.9 (29.11, 8.4 (16.7) and 5.1 (12.9) g S.ha−1.y−1 for mature, secondary, and flooded stands, respectively. Potentiai rates from soil were 0.6 (3.2), 42 (184) and 1.2 (6.1) g S.ha−1.y−l for the same stands. Relative to the 11.7 kg S.ha−1.y−l SO4-S input-output discrepancy that volatile sulphur loss was hypothesized to explain, the observed potential emission rates are at least 200 times too small. Potential emission rates from the leaf litter of the southern Appalachian deciduous forest at Coweet ranged from 0.4 to 2.1 g S.ha−1.y~l and were less than 1 × l0−5 g S.ha−l.y−1 for soil. These rates are too low to account for the SO4-S input-output discrepancy at Coweeta. Potential emissions may be underestimated due to surface adsorption in sampling devices.
Some rainforest legumes emitted sulphur gases from seeds, wood samples, roots, or leaves, or from all these organs. The sulphur gases are ethyl mercaptan and carbon disulphide. These emissions, which are new to plant physiology research, may have community implications as anti-microbial or anti-herbivore agents and, as point sources in a rainforest, may create a sampling problem for ecosystem level studies. Sampling for H2S emissions from plant canopies in the rainforest and at Coweetahas yet to detect this sulphur gas.
Quantifying the contribution of natural sulphur emissions to the atmospheric sulphur burden and to acid rain on a global scale is hampered by the great diversity of habitats, the temporal and spatial variability of sulphur emissions within habitats, and by analytical problems. Sulphur gases exist at low concentrations in nature, consist of numerous chemical species, are transformed from one species to another, and react with analytical surfaces. Quantification of the contribution of natural sulphur emissions to acid rain on a global basis is challenging.
KeywordsLeaf Litter Forest Floor Acid Rain Tropical Rainforest Carbon Disulphide
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