Stimulation of NO and N2O emissions from soils by SO2 deposition

Sulfur dioxide (SO₂) in the atmosphere has been demonstrated to have many adverse impacts on the environment and human health. In this study, deposition of SO₂ ranging from 9.0 to 127.8 mg kg^?1 with an average of 35.7 mg S kg^?1 was found to substantially stimulate NO and N₂O emissions from soils in the humid subtropical areas of Hainan, Fujian, Jiangxi, and Yunnan provinces of China under field conditions. Laboratory tests indicated that the stimulations were mediated biologically as the effects were not observed in sterilized soils. Acidification of soil resulting from SO₂ deposition was not responsible for the stimulated NO and N₂O emissions alone as the stimulation did not occur by acidifying soil with HNO₃ treatment. By using the ¹⁵N tracing method, we found that the N₂O emissions stimulated by SO₂ deposition were from either denitrification, heterotrophic nitrification or both, but not from autotrophic nitrification. Therefore, atmospheric SO₂ deposition would most likely stimulate NO and N₂O emissions in acidic soils in which heterotrophic nitrification dominates NO and N₂O production and waterlogged soils in which denitrification dominates NO and N₂O production.