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NITROGEN SATURATION IN STREAM ECOSYSTEMS
Author(s) -
Earl Stevan R.,
Valett H. Maurice,
Webster Jackson R.
Publication year - 2006
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2006)87[3140:nsise]2.0.co;2
Subject(s) - streams , saturation (graph theory) , river ecosystem , ecosystem , nitrogen , nutrient , phosphorus , ecology , environmental chemistry , tracer , environmental science , chemistry , hydrology (agriculture) , biology , geology , mathematics , computer network , organic chemistry , combinatorics , computer science , physics , geotechnical engineering , nuclear physics
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 ( 15 NO 3 ‐N) to measure uptake. Experiments were conducted in streams spanning a gradient of background N concentration. Uptake increased in four of six streams as NO 3 ‐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.