Earl, S. R.Valett, H. M.Webster, Jackson R.2014-03-272014-03-272006-12Stevan R. Earl, H. Maurice Valett, and Jackson R. Webster 2006. NITROGEN SATURATION IN STREAM ECOSYSTEMS. Ecology 87:3140-3151. http://dx.doi.org/10.1890/0012-9658(2006)87[3140:NSISE]2.0.CO;20012-9658http://hdl.handle.net/10919/46818The concept of nitrogen (N) saturation has organized the assessment of N loading in terrestrial ecosystems. Here we extend the concept to lotic ecosystems by coupling Michaelis-Menten kinetics and nutrient spiraling. We propose a series of saturation response types, which may be used to characterize the proximity of streams to N saturation. We conducted a series of short-term N releases using a tracer ((NO3)-N-15-N) to measure uptake. Experiments were conducted in streams spanning a gradient of background N concentration. Uptake increased in four of six streams as NO3-N was incrementally elevated, indicating that these streams were not saturated. Uptake generally corresponded to Michaelis-Menten kinetics but deviated from the model in two streams where some other growth-critical factor may have been limiting. Proximity to saturation was correlated to background N concentration but was better predicted by the ratio of dissolved inorganic N ( DIN) to soluble reactive phosphorus (SRP), suggesting phosphorus limitation in several high-N streams. Uptake velocity, a reflection of uptake efficiency, declined nonlinearly with increasing N amendment in all streams. At the same time, uptake velocity was highest in the low-N streams. Our conceptual model of N transport, uptake, and uptake efficiency suggests that, while streams may be active sites of N uptake on the landscape, N saturation contributes to nonlinear changes in stream N dynamics that correspond to decreased uptake efficiency.en-USIn CopyrightMichaelis-Mentennitratenitrogen spiralingnitrogen uptakesaturationstable isotopestreamstransient storageforest ecosystemsmountain streamnitrate uptakePhosphorusperiphytonexportlimitationretentionnutrientsArticle - Refereedhttp://www.esajournals.org/doi/pdf/10.1890/0012-9658%282006%2987%5B3140%3ANSISE%5D2.0.CO%3B2https://doi.org/10.1890/0012-9658(2006)87[3140:nsise]2.0.co;2