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

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 (NO{3 ) 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 NO{3 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 NO{3 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. Dissolved organic nitrogen (DON) generally represents a large fraction of the total dissolved nitrogen (N) pool in relatively unpolluted streams (Lewis et al. 1999; Perakis and Hedin 2002), and total organic N (including particulate N) dominates N flux in large rivers across North America (Scott et al. 2007). Agriculture and urban land uses often increase DON concentrations and bioavailability in streams across the United States (Pellerin et al. 2006), although this increase is generally lower than the drastic increases in inorganic N (Stanley and Maxted 2008). It is typically assumed that the majority of DON in streams originates from allochthonous sources such as riparian soils and that the resulting DON pool comprises mainly refractory humic compounds (Aitkenhead-Peterson et al. 2003). Most studies examining stream DON have focused on measuring DON fluxes in stream water or describing allochthonous sources of DON. Because production of dissolved organic carbon (DOC) by in-stream processes can be significant (Kaplan and Bott 1989), DON production may also be significant in streams. Production of autochthonous dissolved organic matter (DOM, a portion of which is DON) by aquatic biota can occur through several mechanisms, including the release of cell constituents via death, senescence, viral lysis, or herbivory (Baines and Pace 1991; Bertilsson and Jones 2003), and these processes occur in both autotrophs (e.g., algae, macrophytes) and heterotrophs (i.e., bacteria, fungi). Additionally, DOM release from aquatic primary producers can occur via passive leakage across cell membranes or active exudation, although the physiological explanation behind this release is not well known (Bertilsson and Jones 2003). Autotrophic DOM production may also vary among phytoplankton taxa (Hellebust 1965), decrease with cell size (MalinskyRushansky and Legrand 1996), or increase in actively growing phytoplankton (Baines and Pace 1991). Aquatic DON production is usually attributed to extracellular release via autotrophs in both marine and lake ecosystems, and we predict that this mechanism will apply in streams as well. * Corresponding author: [email protected] Present address: a National Center for Water Quality Research, Heidelberg University, Tiffin, Ohio b Flathead Lake Biological Station, University of Montana, Polson, Montana c Department of Biology, Ball State University, Muncie, Indiana Limnol. Oceanogr., 58(4), 2013, 1271–1285 E 2013, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2013.58.4.1271