A database of nitrate isotopes in waters of the Northern Forest region in the USA and Canada

Nitrate from atmospheric deposition may be transported to streams without biological uptake in forests, though little is known about the regional extent and variability of these events. There have been many individual studies on one to several catchments. There has been no previous compilation and no comprehensive data archive for this type of catchment data, which has limited generalization about how unprocessed atmospheric nitrate affects soil water, groundwater, and stream water in forests across the region. To advance such an assessment, we compiled water chemistry, nitrate isotope (δ¹⁸O-NO₃^- and δ ¹⁵N-NO₃^-), streamflow, and associated metadata for forested catchments in the Northern Forest Region of the USA and Canada. We introduce this database and show examples of how these data provide insight on transport of unprocessed atmospheric nitrate in catchments. For example, only three of 85 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow; 25 of those 85 streams had high fractions during stormflow. While we note a general lack of information on groundwaters and sparse data on soil waters, the compiled data sometimes show high fractions of unprocessed atmospheric nitrate in near-surface soil waters or groundwaters, but not deep groundwater. Overall, the database helps us to identify the regional extent and variability of fractions of unprocessed atmospheric nitrate in waters of the Northern Forest. In the absence of monitoring for nitrate isotopes in sustained catchment monitoring programs, this information is critical to defining the size and trend of chronic nitrogen pollution effects as well as the efficacy of regulatory and management schemes intended to reduce pollution effects on forests and streams.