title: Nitrogen saturation in stream ecosystems.
abstract: The 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 (15NO3-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.
authors.affiliation: Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0406, USA. Stevan.Earl@asu.edu
authors.forename: Stevan R
authors.initials: SR
authors.lastname: Earl
authors.forename: H Maurice
authors.initials: HM
authors.lastname: Valett
authors.forename: Jackson R
authors.initials: JR
authors.lastname: Webster
mesh_terms: D017753:Ecosystem; D005308:Fertilizers; D005618:Fresh Water / Q000737:chemistry*; D009584:Nitrogen / Q000032:analysis*; D009587:Nitrogen Isotopes
chemical_list: D005308:Fertilizers; D009587:Nitrogen Isotopes; D009584:Nitrogen