『Abstract
We examined the effect of concentration on nitrogen uptake patterns
for a suburban stream in Maryland and addressed the question:
How does NO3- uptake change as
a function of concentration and how do uptake patterns compare
with those found for NH4+? We
applied a longitudinal (stream channel corridor) approach in a
forested stream section and conducted short-term nutrient addition
experiments in late summer 2004. In the downstream direction,
NO3- concentrations decreased
because of residential development in headwaters and downstream
dilution; NH4+ concentrations
slightly increased. The uptake patterns for NO3-
were very different from NH4+.
While NH4+ had a typical negative
relationship between first-order uptake rate constant (Kc) and stream size, NO3-
had a reverse pattern. We found differences for other metrics,
including uptake velocity (Vf) and areal
uptake rate (U). We attributed these differences to a stream size
effect, a concentration effect and a biological uptake capacity
effect. For NO3- these combined
effects produced a downstream increase in Kc,
Vf and U; for NH4+
they produced a downstream decrease in Kc
and Vf, and a not well defined pattern for
U. We attributed a downstream increase in NO3-
uptake capacity to an increase in hyporheic exchange and a likely
increase in carbon availability. We also found that Kc
and Vf were indirectly related with concentration.
similar evidence of ‘nutrient saturation’ has been reported in
other recent studies. Our results suggest that higher-order uptake
models might be warranted when scaling NO3-
uptake across watersheds that are subject to increased nitrogen
loading.
Keywords: Nitrogen; Nutrient additions; Nutrient saturation; Streams』
Introduction
Study area
Methods
Nutrient addition experiments
Data analysis
Transport and transient storage
Uptake
Uptake metrics
Results
Physical and hydrologic characteristics
Chemical characteristics
Comparison of uptake metrics against other studies
Spatial patterns of uptake metrics
Concentration and uptake metrics
Discussion
Spatial patterns of uptake
Effects of concentration and biological uptake capacity
Implications for scaling
Conclusions
Acknowledgments
Open Access
References