Nitrogen deposition increases NO emission from an N-saturated subtropical forest in southwest China.

Nitrous oxide (NO) is a major greenhouse gas, with elevated emission being reported from subtropical forests that receive high nitrogen (N) deposition. After 10 years of monthly addition of ammonium nitrate (NHNO) or sodium nitrate (NaNO) to a Mason pine forest at Tieshanping, near Chongqing city in Southwest China, the simulated N deposition was stopped in October 2014. The results of soil NO emissions monitoring in different seasons during the nitrogen application period showed that nitrogen addition significantly increased soil NO emission. In general, the NO emission fluxes were positively correlated to nitrate (NO) concentrations in soil solution, supporting the important role of denitrification in NO production, which was also modified by environmental factors such as soil temperature and moisture. After stopping the application of nitrogen, the soil NO emissions from the treatment plots were no longer significantly higher than those from the reference plots, implying that a decrease in nitrogen deposition in the future would cause a decrease in NO emission. Although the major forms of N deposition, NH and NO, had not shown significantly different effects on soil NO emission, the reduction in NH deposition may decrease the NO concentrations in soil solution faster than the reduction in NO deposition, and thus be more effective in reducing NO emission from N-saturated forest soil in the future.