Nitrogen deposition increases N2O emission from an N-saturated subtropical forest in southwest China.

Nitrous oxide (N2O) 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 (NH4NO3) or sodium nitrate (NaNO3) 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 N2O emissions monitoring in different seasons during the nitrogen application period showed that nitrogen addition significantly increased soil N2O emission. In general, the N2O emission fluxes were positively correlated to nitrate (NO3-) concentrations in soil solution, supporting the important role of denitrification in N2O production, which was also modified by environmental factors such as soil temperature and moisture. After stopping the application of nitrogen, the soil N2O 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 N2O emission. Although the major forms of N deposition, NH4+ and NO3-, had not shown significantly different effects on soil N2O emission, the reduction in NH4+ deposition may decrease the NO3- concentrations in soil solution faster than the reduction in NO3- deposition, and thus be more effective in reducing N2O emission from N-saturated forest soil in the future.