Nitrogen air pollution concerns for Nepal’s forested ecosystems and lichen bioindicators

Excessive reactive nitrogen (Nr) is a growing challenge for sensitive terrestrial habitats like forests. Nepal is experiencing threats of nitrogen (N) pollution largely transported from the Indo-Gangetic Plain (IGP) – a global nitrogen pollution hotspot – which affects biodiversity, ecosystem functioning and human health. This urges the quantification of the impacts of those pollutants in the Himalayan region. Our analysis, based on the atmospheric chemistry transport model (European Monitoring and Evaluation Programme-Weather Research and Forecasting 2010 emission with 2018 chemistry and meteorology) with land use land cover and digital elevation model, shows that 95-99% of Nepal’s forests have already exceeded the United Nations Economic Commission for Europe (UNECE)-recommended ammonia (NH₃) critical levels and N critical loads. Overall, ammonia (NH₃) (0.54-13.13 μg m^–3) and nitrogen oxides (NO_x; 0.05-13.64 μg m^–3) concentrations are higher at low elevation forests, but a contrasting pattern of bulk N deposition (4.52-38.56 kg N ha^–1 yr^–1) is observed in forests along the elevation gradients and forest types. Wet deposition of N is exceptionally high in forested areas receiving high precipitation, but dry deposition is heterogeneously distributed over different parts of the country. The forests in the lowland Tarai and Mid-hills that are near IGP are exposed to high concentrations of NH₃ and NO_x – thus are at a higher risk of biodiversity loss. Contributing only small shares, deciduous and needleleaf forests are vulnerable to N pollution as they cover the subtropical to subalpine region of the Mid-hills and host most of the sensitive species like lichens. This demonstrates a serious concern of N pollution on biodiversity and ecosystem services in the region. The empirical testing of N impacts on Nepal’s forested ecosystems is now crucial to establish the field-based toxicity threshold of N-based pollutants for biodiversity conservation and policy negotiation.