Quantifying ambient nitrogen uptake and functional relationships of uptake versus concentration in streams: a comparison of stable isotope, pulse, and plateau approaches Matt T. Trentman . Walter K. Dodds . Jane S. Fencl . Kayla Gerber . Jay Guarneri . Sean M. Hitchman . Zach Peterson . Janine Ru ¨egg Received: 24 June 2014 / Accepted: 28 May 2015 / Published online: 5 June 2015 Ó Springer International Publishing Switzerland 2015 Abstract Nutrient releases and spiraling metrics are frequently used to quantify the downstream transport of nutrients and to better understand the effects of anthropogenic inputs to downstream waters. Ambient uptake rates in streams can be measured through stable isotope enrichments, while pulse and plateau additions can estimate such rates via extrapolation and modeling techniques, respectively. Data from these releases can be used to estimate ambient uptake rates from nutrient additions and possibly determine the functional rela- tionships between nutrient concentrations and uptake rates. Here, we compared estimated ambient rates calculated from established pulse and plateau approaches, results obtained from new modeling approaches, and rates at ambient concentrations from stable isotope enrichments. Comparative releases of NH 4 Cl and 15 NH 4 Cl were conducted in four experi- mental reaches across the grassland Kings Creek and urban Campus Creek, KS. Nutrient uptake was predominantly linear with increasing ammonium. Estimated ambient uptake rates varied among sites, release methods, and data analysis approaches. However, plateau ambient rates from new modeling approaches matched closely with measured ambient rates from isotope enrichments at three sites, suggest- ing that modeled plateau data may be best for a first look at determining nutrient uptake rates at an individual site. Limitations and benefits of each approach vary; however, baseflow discharge may be a key driver when choosing a method. If possible, multiple methods should be attempted at each location and under each novel set of conditions to determine the best approach prior to designing and implementing a more extensive series of measurements. Keywords Nutrient spiraling  TASCC  Uptake length  Ammonium uptake Introduction In-stream nutrient uptake and other nutrient cycling processes dictate the downstream movement of nutri- ents, especially those derived from terrestrial runoff and ground water input (e.g. Peterson et al. 2001; Mulholland et al. 2008). Quantifying nutrient uptake, its functional relationship with nutrient concentra- tions, and its effect on downstream transport is important for assessing the effects of nutrient enhance- ment by anthropogenic sources (Bernot and Dodds 2005). Specifically, nitrogen delivery to coastal areas from agriculturally impacted watersheds can cause Responsible Editor: Jacques C Finlay. M. T. Trentman (&)  W. K. Dodds  J. S. Fencl  K. Gerber  J. Guarneri  S. M. Hitchman  Z. Peterson  J. Ru ¨egg Division of Biology, Kansas State University, Manhattan, KS 66506, USA e-mail: mtrentman@ksu.edu 123 Biogeochemistry (2015) 125:65–79 DOI 10.1007/s10533-015-0112-5