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

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 (K c ) and stream size, NO3 − had a reverse pattern. We found differences for other metrics, including uptake velocity (V f ) 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 K c , V f and U; for NH4 + they produced a downstream decrease in K c and V f , 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 K c and V f 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.