We identified variables controlling stream nitrogen concentrations and fluxes, and how they have changed over time, by synthesizing 20 time series ranging from 5 to 51 years of data collected from forest and grassland dominated watersheds across Europe, North America, and East Asia and across four climate types (tropical, temperate, Mediterranean, and boreal) using the International Long-Term Ecological Research Network. We found declining trends in bulk ammonium and nitrate deposition, with ammonium contributing significantly more to atmospheric nitrogen deposition over time. Among sites, there were significant positive relationships between (1) precipitation and stream ammonium and nitrate fluxes and (2) atmospheric nitrogen inputs and stream nitrogen concentrations and fluxes. There were no significant relationships between air temperature and stream nitrogen export. Our long-term data shows that although nitrogen deposition is declining over time, atmospheric nitrogen inputs and precipitation remain the main predictors for nitrogen exported from forested and grassland watersheds. Overall, we also demonstrate that long-term monitoring provides understanding of ecosystems and biogeochemical cycling that would not be possible with short-term studies alone. Acknowledgements: We thank the organizers and funders of the ILTER Nitrogen Initiative Training Course and Workshop in Hokkaido, Japan in June 2016, which brought together many of the participants in this project. Templer was supported by a US National Science Foundation LTER grant NSF DEB 1637685. McDowell was supported by US National Science Foundation LTER grant NSF DEB 1831592. We are grateful to the EU Horizon 2020 funded eLTER PLUS project (Grand Agreement No. 871128) for financial support to Haase and Dirnböck. Dirnböck was also funded by the LTER-CWN project (FFG project number 858024). This study was partly supported by the Research Initiative Grants of the ILTER, Grants-in-Aid for Scientific Research (17H03833), and Research Institute for Humanity and Nature (RIHN; a constituent member of NIHU) Project No. 14200156. Sharif was supported by NRT-INFEWS: UMD Global STEWARDS (STEM Training at the Nexus of Energy, WAter Reuse and FooD Systems) that was awarded to the University of Maryland School of Public Health by the National Science Foundation National Research Traineeship Program, Grant number 1828910. The monitoring of the Svartberget site in Sweden was funded by the SITES program from the Swedish Research Council. The monitoring of the Volbu Nyhaga site in Norway was part of JOVA - The Norwegian Agricultural Environmental Monitoring Programme, financed by the Ministry of Agriculture and Food. Lajtha was supported by NSF grants DEB-1257032 and DEB-1440409 to the H. J. Andrews Long Term Ecological Research program. The data collection in the Wüstebach catchment was supported by TERENO (Terrestrial Environmental Observatories) funded by the Helmholtz-Gemeinschaft. We thank the “Hessisches Landesamt für Naturschutz, Umwelt und Geologie” for providing data from the Rhine-Main-Observatory. I.Kokorīte was supported by the University of Latvia grant No. AAp2016/B041//Zd2016/AZ03. We thank the Latvian Environment, Geology and Meteorology Center for providing the monitoring data for Latvian site. Anna Avila was supported by Spanish Ministry of Science projects CGL2017-84687-C2-2-R and CGL2009-13188-C03-01. We thank the Tyrolean Alps Long-Term Sociological Ecological and Research (LTSER, Austria) and R. Psenner and S. Morales for helping with data for Piburger See. Data from Hubbard Brook were supported by the National Science Foundation (DEB-1907683) and US Forest Service, Northern Research Station. A. Robison was supported by US National Science Foundation LTER grant NSF OCE 1637630. W. Dodds was supported by NSF DEB 2025849. Observations at Lange Bramke site were funded by the Ministry of Nutrition, Agriculture and Consumer Protection of Lower Saxony under the Permanent Soil Monitoring Programme. The Krofdorf site was funded by the Hessian Ministry of Environment, Climate Protection, Agriculture and Consumer Protection under the “Waldökosystemstudie Hessen”. SRM Lins was supported by the São Paulo Research Foundation - FAPESP, grant number 2012/20377-9. Long-term monitoring at Lake Maggiore (LTER site EU-IT08-001-A) was funded by the International Commission for the Protection of Swiss-Italian waters (CIPAIS).

2022