Nitrogen emissions from agriculture, traffic, and industry can harm terrestrial biodiversity, and this chapter measures these effects using Mean Species Abundance (MSA). When ammonia (NH₃) and nitrogen oxides (NO_x) enter the atmosphere, they eventually deposit onto ecosystems, altering soils and shifting species composition. These changes favor nitrogenloving species and reduce the abundance of many native plants. The GLOBIO model estimates how much MSA is lost per kilogram of emissions from each country by tracing where emitted nitrogen ends up. Sourcereceptor matrices show how airborne nitrogen travels across regions, linking emissions in one place to biodiversity impacts elsewhere. The model calculates separate impact factors for NH₃ and NOₓ because they behave differently in the atmosphere. Each factor expresses how emissions reduce plant abundance over time and area, making MSA the central measure of impact rather than PDF. Universal MSA relationships are used, which introduces uncertainties due to ecological variation among regions. Nitrogenion conversion factors ensure consistent comparison between emission types.