Significant contributions of combustion‐related NH 3 and non‐fossil fuel NO x to elevation of nitrogen deposition in southwestern China over past five decades

Anthropogenic nitrogen (N) emissions and deposition have been increasing over past decades. However, spatiotemporal variations of N deposition levels and major sources remain unclear in many regions, which hinders making strategies of emission mitigation and evaluating effects of elevated N deposition. By investigating moss N contents and δ 15 N values in southwestern (SW) China in 1954–1964, 1970–1994, and 2005–2015, we reconstructed fluxes and source contributions of atmospheric ammonium ( NH 4 + ) and nitrate ( NO 3 ‐ ) deposition and evaluated their historical changes. For urban and non‐urban sites, averaged moss N contents did not differ between 1954–1964 and 1970–1994 (1.2%–1.3%) but increased distinctly in 2005–2015 (1.6%–2.3%), and averaged moss δ 15 N values decreased from +0.4‰ to +3.3‰ in 1954–1964 to −1.9‰ to −0.7‰ in 1974–1990, and to −4.8‰ to −3.6‰ in 2005–2015. Based on quantitative estimations, N deposition levels from the 1950s to the 2000s did not change in the earlier 20 years but were elevated substantially in the later 30 years. Moreover, the elevation of NH 4 + deposition (by 12.2 kg‐N/ha/year at urban sites and 4.6 kg‐N/ha/year at non‐urban sties) was higher than that of NO 3 ‐ deposition (by 6.0 and 2.9 kg‐N/ha/year, respectively) in the later 30 years. This caused a shifted dominance from NO 3 ‐ to NH 4 + in N deposition. Based on isotope source apportionments, contributions of combustion‐related NH 3 sources (vehicle exhausts, coal combustion, and biomass burning) to the elevation of NH 4 + deposition were two times higher than volatilization NH 3 sources (wastes and fertilizers) in the later 30 years. Meanwhile, non‐fossil fuel NO x sources (biomass burning, microbial N cycles) contributed generally more than fossil fuel NO x sources (vehicle exhausts and coal combustion) to the elevation of NO 3 ‐ deposition. These results revealed significant contributions of combustion‐related NH 3 and non‐fossil fuel NO x emissions to the historical elevation of N deposition in SW China, which is useful for emission mitigation and ecological effect evaluation of atmospheric N loading.