Johnson Laura T.

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Laura T.

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  • Preprint
    Stream denitrification across biomes and its response to anthropogenic nitrate loading
    ( 2007-06-06) Mulholland, Patrick J. ; Helton, Ashley M. ; Poole, Geoffrey C. ; Hall, Robert O. ; Hamilton, Stephen K. ; Peterson, Bruce J. ; Tank, Jennifer L. ; Ashkenas, Linda R. ; Cooper, Lee W. ; Dahm, Clifford N. ; Dodds, Walter K. ; Findlay, Stuart E. G. ; Gregory, Stanley V. ; Grimm, Nancy B. ; Johnson, Sherri L. ; McDowell, William H. ; Meyer, Judy L. ; Valett, H. Maurice ; Webster, Jackson R. ; Arango, Clay P. ; Beaulieu, Jake J. ; Bernot, Melody J. ; Burgin, Amy J. ; Crenshaw, Chelsea L. ; Johnson, Laura T. ; Niederlehner, B. R. ; O'Brien, Jonathan M. ; Potter, Jody D. ; Sheibley, Richard W. ; Sobota, Daniel J. ; Thomas, Suzanne M.
    Worldwide, anthropogenic addition of bioavailable nitrogen (N) to the biosphere is increasing and terrestrial ecosystems are becoming increasingly N saturated, causing more bioavailable N to enter groundwater and surface waters. Large-scale N budgets show that an average of about 20-25% of the N added to the biosphere is exported from rivers to the ocean or inland basins, indicating substantial sinks for N must exist in the landscape. Streams and rivers may be important sinks for bioavailable N owing to their hydrologic connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favor microbial denitrification. Here, using data from 15N tracer experiments replicated across 72 streams and 8 regions representing several biomes, we show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of instream nitrate that is removed from transport. Total uptake of nitrate was related to ecosystem photosynthesis and denitrification was related to ecosystem respiration. Additionally, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.
  • Article
    Quantifying the production of dissolved organic nitrogen in headwater streams using 15N tracer additions
    (Association for the Sciences of Limnology and Oceanography, 2013-07) Johnson, Laura T. ; Tank, Jennifer L. ; Hall, Robert O. ; Mulholland, Patrick J. ; Hamilton, Stephen K. ; Valett, H. Maurice ; Webster, Jackson R. ; Bernot, Melody J. ; McDowell, William H. ; Peterson, Bruce J. ; Thomas, Suzanne M.
    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 () 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 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 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.