Nitrogen saturation in stream ecosystems

Stevan R. Earl, H. Maurice Valett, Jackson R. Webster

Research output: Contribution to journalArticlepeer-review

131 Scopus citations

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.

Original languageEnglish
Pages (from-to)3140-3151
Number of pages12
JournalEcology
Volume87
Issue number12
DOIs
StatePublished - Dec 2006

Keywords

  • Michaelis-Menten
  • Nitrate
  • Nitrogen spiraling
  • Nitrogen uptake
  • Saturation
  • Stable isotope
  • Streams

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