Schmidt, G., May, C., Gebauer, G., & Schulze, E. D. (1996). Uptake of [15N]ammonium
and 15N]nitrate in a 140-year-old spruce stand (Picea abies) in the Fichtelgebirge (NE-Bavaria).
Isotopes in Environmental and Health Studies, 32, 141-148. doi:10.1080/10256019608036305.