Stimulation of NO and N₂O emissions from soils by SO₂ deposition

Sulfur dioxide ( SO 2 ) in the atmosphere has been demonstrated to have many adverse impacts on the environment and human health. In this study, deposition of SO 2 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 N 2 O 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 SO 2 deposition was not responsible for the stimulated NO and N 2 O emissions alone as the stimulation did not occur by acidifying soil with HNO 3 treatment. By using the 15 N tracing method, we found that the N 2 O emissions stimulated by SO 2 deposition were from either denitrification, heterotrophic nitrification or both, but not from autotrophic nitrification. Therefore, atmospheric SO 2 deposition would most likely stimulate NO and N 2 O emissions in acidic soils in which heterotrophic nitrification dominates NO and N 2 O production and waterlogged soils in which denitrification dominates NO and N 2 O production.