Factors affecting ammonium uptake in streams – an inter‐biome perspective

Abstract

Summary 1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter‐biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole‐stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of 15N‐ammonium and downstream measurements of 15N‐ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in‐stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake.

The saturation of N cycling in Central Plains streams: 15N experiments across a broad gradient of nitrate concentrations

“…Biomass-specific uptake was calculated as total molar mass of 15 N tracer per m 2 of each compartment divided by mole frequency of 15 N to 14 N in the NO 3 -of the overlying stream water. Biological N demand was calculated for each stream according to the method and assumptions of Webster et al (2003). Autotrophic demand was calculated for each stream, based on GPP measurements from stream metabolism estimates.…”

“…Heterotrophic N demand was then calculated using CR from the metabolism estimates (Webster et al 2003). CR was corrected for autotrophic respiration (30% of GPP) and nitrification (2 mol O 2 /mol N) to determine heterotrophic respiration.…”

Land use influences stream nitrate uptake in the Lake Taupo catchment

“…leaf decomposition) and autotrophic (i.e. algal growth) processes in streams (Elwood et al 1981;Tank & Dodds 2003). However, to our knowledge, few stream studies have demonstrated a link between whole-stream N uptake and dissolved phosphorus availability.…”

“…However, to our knowledge, few stream studies have demonstrated a link between whole-stream N uptake and dissolved phosphorus availability. Webster et al (2003) reported a negative relationship between stream nitrate uptake and streamwater N:P ratio but they also noted that relationship was not significant when data from one of their sites was excluded or when a rank correlation was performed. In terrestrial systems, phosphorus limitation has been shown to reduce NO 3 ( uptake in plants (Lee 1982;Rufty et al 1990) so it is likely that phosphorus availability could also influence NO 3 ( uptake by instream biota.…”

The Lotic Intersite Nitrogen Experiments: an example of successful ecological research collaboration

“…This "aha moment" in the historical origins of the LINX collaboration illustrates how scientific progress often results from the juxtaposition of tools and concepts from diverse fields (Fisher 1997). Subsequently, the LINX projects investigated spiraling of NH 4 + , NO 3 -, organic N, and gaseous forms of N. In all phases of this work, we tried to link our measurements with hydrologic foundations (e.g., N uptake length [S w ] linked with hydraulic retention, subsurface area, and hydraulic uptake length; Webster et al 2003).…”