Measurement report: Soil reactive nitrogen gas emissions from the Tibetan Plateau

Abstract. The Tibetan Plateau, highly sensitive to climate change, exerts strong atmospheric oxidation capacity partly through rapid cycling of atmospheric reactive nitrogen (Nr). Soil Nr emissions play a crucial role in atmospheric nitrogen cycling and oxidation capacity, yet their emissions on the Tibetan Plateau remain poorly quantified. Combining dynamic chamber measurements, laboratory analysis, and a parameterized model, we assessed the characteristics, driving factors, spatial distribution and annual emissions of soil Nr in the Tibetan Plateau. We found that the optimum soil fluxes of nitrous acid (HONO), nitric oxide (NO), nitrogen dioxide (NO₂), and ammonia (NH₃) were 21.6 ± 8.4, 43.7 ± 14.7, 15.8 ± 1.3, and 190.0 ± 116.3 ng N m⁻² s⁻¹, respectively. These emissions were mainly influenced by soil pH, nutrient content, and microbial community composition. After exposure to atmospheric NO_x and ozone (O₃), Nr emissions from forest soils were enhanced but those from croplands and grasslands were suppressed. The estimated annual emissions of soil HONO, NO, and NO_x from Tibetan Plateau to be 7.0 ± 3.4 Gg N yr⁻¹, 11.6 ± 7.8 Gg N yr⁻¹, and 20.3 ± 7.0 Gg N yr⁻¹, respectively. Soil HONO emissions contribute approximately 10.5 % of the external (NO_x-independent) daytime atmospheric HONO sources and modulate the regional atmospheric chemical balance by elevating the HONO/NO_x ratio. Our results provide the first integrated quantification of soil Nr emissions on the Tibetan Plateau and emphasize their importance for regional nitrogen cycling and atmospheric oxidation capacity.