Sulfur pollution suppression of the wetland methane source in the 20th and 21st centuries.

Natural wetlands form the largest source of methane (CH(4)) to the atmosphere. Emission of this powerful greenhouse gas from wetlands is known to depend on climate, with increasing temperature and rainfall both expected to increase methane emissions. This study, combining our field and controlled environment manipulation studies in Europe and North America, reveals an additional control: an emergent pattern of increasing suppression of methane (CH(4)) emission from peatlands with increasing sulfate (SO(4)(2-)-S) deposition, within the range of global acid deposition. We apply a model of this relationship to demonstrate the potential effect of changes in global sulfate deposition from 1960 to 2080 on both northern peatland and global wetland CH(4) emissions. We estimate that sulfur pollution may currently counteract climate-induced growth in the wetland source, reducing CH(4) emissions by approximately 15 Tg or 8% smaller than it would be in the absence of global acid deposition. Our findings suggest that by 2030 sulfur pollution may be sufficient to reduce CH(4) emissions by 26 Tg or 15% of the total wetland source, a proportion as large as other components of the CH(4) budget that have until now received far greater attention. We conclude that documented increases in atmospheric CH(4) concentration since the late 19th century are likely due to factors other than the global warming of wetlands.

Growth, production and interspecific competition in Sphagnum: effects of temperature, nitrogen and sulphur treatments on a boreal mire.

• Growth and production of Sphagnum balticum and interspecific competition between S. balticum and either Sphagnum lindbergii or transplanted Sphagnum papillosum, were studied in a 4-yr field experiment in a poor fen. • Temperature and influxes of nitrogen (N) and sulphur (S) were manipulated in a factorial design. The mean daily air temperature was increased by 3.6°C with glasshouse enclosures. Nitrogen loads were increased 15-fold and S loads seven-fold compared with the natural loads up to influxes observed during the 1980s in south-western Sweden. • Production of S. balticum decreased with increasing temperature and N-influx. The N treatment significantly reduced the incremental length of S. balticum, and this reduction was reinforced with time (24% in the first year to 51% in the final year). The area covered by S. lindbergii changed with time in all treatments and S. papillosum area increased significantly in the temperature-treated plots. • Growth, production and competitive patterns change if the environmental conditions change. Increased N deposition and raised temperature may transform mires currently dominated by Sphagnum into vascular-plant-dominated mires.