Abstract
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 δ15 N values in southwestern China in 1954-1964, 1970-1994, and 2005-2015, we reconstructed fluxes and source contributions of atmospheric ammonium (NH4 + ) and nitrate (NO3 - ) deposition and their historical changes. Moss N contents did not differ between 1954-1964 and 1970-1994 (1.2%-1.3%) but increased distinctly in 2005-2015 (1.6%-2.2%) on average. Moss δ15 N values decreased from +0.4‰-+3.3‰ in 1954-1964 to -1.9‰--0.7‰ in 1974-1990, and to -2.9‰ in 2005-2015 on average. 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 NH4 + deposition (by 12.2 kg-N/ha/yr at urban sites and 4.6 kg-N/ha/yr at non-urban sties) was higher than that of NO3 - deposition (by 6.0 kg-N/ha/yr and 2.9 kg-N/ha/yr, respectively) in the later 30 years. This caused a shifted dominance from NO3 - to NH4 + in N deposition. Based on isotope source apportionments, contributions of combustion NH3 sources (vehicle exhausts, coal combustion, biomass burning) to the elevation of NH4 + deposition were two times higher than volatilization NH3 sources (wastes and fertilizers) in the later 30 years. Meanwhile, non-fossil fuel NOx sources (biomass burning, microbial N cycles) contributed generally more than fossil fuel NOx sources (vehicle exhausts, coal combustion) to the elevation of NO3 - deposition. These results revealed significant contributions of combustion NH3 and non-fossil fuel NOx emissions to the historical elevation of N deposition in southwestern China, which is useful for emission mitigation and ecological effect evaluation of atmospheric N loading.
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