There are many reasons that the isotopic signature of deposited N is unlikely to be causing the declines in plant δ15N. Although our analysis of foliar δ15N was limited to after 1980, declines in foliar, tree-ring and sediment δ15N pre-date the onset of widespread inorganic N fertilizer use, as well as any timing ofshifts to more reduced forms of N in deposition. Also, despite variation in the isotopic signatures of N deposition and its sources, there is no evidence currently that the signature of atmospheric N deposition has been declining overtime. A global, comprehensive dataset on the signature of N deposition does not exist and would be helpful to generate. Yet, even if the isotopic signature of N deposition has been declining, changes in N availability can have a stronger influence on plant δ15N than the signature of added N.

Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia de Santa Cruz. Universidad Tecnológica Nacional. Facultad Regional Santa Cruz. Centro de Investigaciones y Transferencia de Santa Cruz. Universidad Nacional de la Patagonia Austral. Centro de Investigaciones y Transferencia de Santa Cruz; Argentina

Fil: Guerrieri, Rossella. Centre for Ecological Research and Forestry Applications; España

Fil: Gundale, Michael J.. Swedish University of Agricultural Sciences; Suecia

Fil: Hietz, Peter. University of Natural Resources and Life Sciences; Austria

Fil: Werner, Christiane. Albert Ludwigs University of Freiburg; Alemania

Fil: Fang, Yunting. Chinese Academy of Sciences; República de China

Fil: Elmore, Andrew J.. University of Maryland; Estados Unidos

Fil: Nelson, David M.. University of Maryland; Estados Unidos

Fil: Templer, Pamela H.. Boston University; Estados Unidos