Global impacts of sulfate deposition from acid rain on methane emissions from natural wetlands

Natural wetlands form the largest methane (CH4) source to the atmosphere. A collection of recent field and laboratory studies point to an anthropogenic control on CH4 emissions from these systems: acid rain sulfate (SO42-) deposition. These studies ranging from the UK, USA, Canada, Sweden and Czech Republic demonstrate that low rates of SO42- deposition, within the range commonly experienced in acid rain impacted regions, can suppress CH4 emissions by as much as 40% and that the response of CH4 emissions to increasing rates of SO42- deposition closely mirrors changes in sulfate reduction rates with SO42- deposition. This indicates that the suppression in CH4 flux is the result of acid rain stimulating a competitive exclusion of methanogenesis by sulfate reducing bacteria, resulting in reduced methane production. These findings were extrapolated to the global scale by combining modelled, spatially explicit data sets of CH4 emission from wetlands across the globe with modelled S deposition. Results indicate that this interaction may be important at the global scale, suppressing CH4 emissions from wetlands in 2030 by as much as 20--28Tg, and, in the process, offsetting predicted climate induced growth in the wetland CH4 source