Fates of Atmospheric Reactive Nitrogen Emissions From Polluted Regions

Fates of atmospheric reactive nitrogen (N) mainly contain foliar absorption of gaseous N, deposition in emission regions, and transportation out of emission regions, which determines their eco-environmental and climatic effects. However, it has long been challenging to estimate fractions and fluxes of reactive N fates in the atmosphere. Here we quantified regional ammonium-N and nitrate-N deposition in North America, Europe, and East Asia based on moss N contents and natural N isotopes. In combination with regional N emissions and foliar absorption rates of N gases, we further constrained major fates (deposition, foliar absorption, and export) of N emissions in the above regions. We found that 72% ± 15%, 59% ± 8%, and 72% ± 11% of N emissions (15.7 ± 4.3, 25.5 ± 5.3, and 28.0 ± 6.1 kg-N/ha/year) deposited back to three study regions, respectively. Having larger N emissions and deposition, Europe and East Asia also had larger exports (5.9 ± 5.9 and 4.2 ± 6.4 kg-N/ha/year, respectively) than those of North America (0.6 ± 4.8 kg-N/ha/year). Adding the area-weighted N emissions in three regions together, about 2/3 of N emissions deposited back to land, 1/6 was absorbed by plant leaves, and the remaining 1/6 was transported out of three regions. This work provides a unique method for unlocking processes of land-air N exchanges and useful evidence for evaluating effects of regional atmospheric N pollution.