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Soils have been widely acknowledged as important natural sources of nitric oxide (NO) and meanwhile sinks of nitric dioxide (NO

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.

The emission of nitrogen oxides (NOx) in China decreased by 15% from 2010 to 2015 (without a significant decrease in NH3 emission), resulting in the decline of nitrogen (N) deposition in East Asia. Empirical N critical load exceedance was used to assess the benefit of the NOx emission reduction in China to natural ecosystems in East Asia. Empirical N critical loads for major forest and grassland types in East Asia were assigned based on field manipulation experiments for N effects. The critical load map based on the minimum of the critical load range of each vegetation type showed that empirical critical loads were generally lower in the Tibetan Plateau and some parts of northeastern China (≤5 kgN·ha−1·a−1), and higher in northern and southern China (≥20 kgN·ha−1·a−1). Empirical critical loads were also low in some parts of central and northern Japan (≤5 kgN·ha−1·a−1) and the south Korean Peninsula (5–10 kgN·ha−1·a−1). As a benefit of NOx emission reduction in China, N deposition in East Asia decreased significantly from 2010 to 2015. The total area and total amount of critical load exceedance in East Asia declined 4.6% and 14.3% respectively, suggesting great benefits to natural ecosystems. Keywords: Nitrogen deposition, Air pollution effect, Fertilization experiment, CMAQ model, Emission abatement, Nitrogen saturation

Nitrous oxide (N