Quantifying the production of dissolved organic nitrogen in headwater streams using 15N tracer additions

Laura Johnson was a contributing author, before starting her career at NCWQR. ABSTRACT Most nitrogen (N) assimilation in lake and marine ecosystems is often subsequently released via autochthonous dissolved organic nitrogen (DON) production, but autochthonous DON production has yet to be quantified in flowing waters. We measured in-stream DON production following 24 h 15N-nitrate (NO3-) tracer additions in 36 headwater streams, a subset of sites from the second Lotic Intersite Nitrogen eXperiment. Streams were located in five North American ecoregions and drained basins dominated by native vegetation, agriculture, or urban land use. Using a two-compartment model, we could quantify DON production in 15 streams as a function of DO15N derived from 15N tracer in biomass compartments. The streams with detectable DON production had higher % modified land use (agriculture + urban) in their basins than did streams with undetectable DON production. Median DON production represented 8% of total NO3- uptake when we used N biomass estimates based on N assimilated over 1 d (measured directly from the 15N additions). Median DON production was 17% of total NO3- uptake when we used N assimilated over 42 d (extrapolated from previous 15N tracer studies). Variation in DON production was positively correlated with ecosystem respiration, indicating that stream heterotrophy may influence DON production. In-stream DON production was similar in magnitude to stream denitrification and nitrification, indicating that the production of autochthonous DON can represent a substantial transformation of stream N. Our results confirm that headwater streams can quickly convert inorganic N into organic forms, although the ultimate fate of DON remains unclear.We thank the National Science Foundation (NSF) Long Term Ecological Research network, the U.S. Forest Service, the National Park Service, local municipalities, and the many private landowners who granted us access to their property. We also thank the many students and scientists who helped with fieldwork and laboratory analyses that contributed to the LINX II project and especially those associated with the subset of regions included in this study. Reviews by Emily Bernhardt, Natalie Griffiths, Mia Stephen, and an anonymous reviewer greatly improved earlier versions of this manuscript. This work was supported by a grant from the National Science Foundation Division of Environmental Biology (NSF DEB-0111410), and L. Johnson was partially supported by fellowships from the Center for Aquatic Conservation at the University of Notre Dame and the Bayer Corporation while preparing this manuscript. This article is dedicated to the memory of Patrick J. Mulholland, whose exceptional leadership ensured the success of the LINX II project. His inspiration to the field of stream biogeochemistry has been immeasurable and his support and collaboration will be sorely missed