Longitudinal assessment of the effect of concentration on stream N uptake rates in an urbanizing watershed

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.