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In many countries worldwide, NO x emissions currently decrease as a result of pollution control, while NH 3 emissions stagnate or continue to increase. Little is known about horizontal deposition of NO 3 - and NH 4 + , the oxidation/neutralization products of these primary pollutants. To close the knowledge gap, we studied atmospheric inputs of NO 3 - and NH 4 + at two mountain-top sites near the Czech-German-Polish borders during winter. Horizontal deposition via ice accretions (rime) made up 26-30 % of total atmospheric input of reactive nitrogen (N r ). Such high horizontal depositions should not be neglected in ecosystem N studies which currently often consider only vertical deposition via snow. Snow nitrate N was the largest type of N r deposition (40-52 %), with snow ammonium N being the second largest (20-30 %). Rime ammonium N contributed a similar amount to total N r input as rime nitrate N (12-16 %). The total inorganic N r deposition was 4-6 kg ha -1  winter -1 . Across the sites, the mean δ 15 [Formula: see text] and δ 15 [Formula: see text] values fell in a relatively narrow range from -3.1 to -7.3 ‰. Three systematic isotope patterns were observed: (i) NH 4 + -N was always heavier in rime than in snow, (ii) NO 3 - -N was always heavier in rime than in snow, and (iii) NO 3 - -N was always heavier than NH 4 + -N. For source apportionment, the Bayesian isotope mixing model SIMMR was used. Counter-intuitively, vehicles were larger sources of NH 3 in rime than volatilation from animal waste plus fertilizers (46 vs. 19 %). The largest NO 3 - contributions to rime were derived from vehicles and biomass burning, followed by natural gas combustion and coal burning in power plants and households. Natural gas represented the largest source of nitrate in snow. Nitrate sources appeared to be better-mixed than ammonium sources. Our isotope-based source apportionment differed from national emission inventories, offering original insights into local atmospheric N r inputs., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Martin Novak reports financial support was provided by Czech Science Foundation. Iva Hunova reports financial support was provided by Technology Agency of the Czech Republic. Radim Seibert reports financial support was provided by Technology Agency of the Czech Republic. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 Elsevier B.V. All rights reserved.)