Continuous In-Stream Assimilatory Nitrate Uptake from High-Frequency Sensor Measurements

Recently developed in situ NO<sub>3</sub><sup>–</sup> sensors provide new opportunities to measure changes in stream concentration at high temporal frequencies that historically have not been feasible. In this study, we used multiparameter sensor measurements to relate assimilatory NO<sub>3</sub><sup>–</sup> uptake to metabolic rates and calculate continuous uptake rates for two stream reaches and a whole stream network. Two years of continuous 15 min data from a forest and agricultural reach of the Selke river (456 km<sup>2</sup>) revealed a strong correlation between assimilatory NO<sub>3</sub><sup>–</sup> uptake and growth primary production (GPP) for the forest (<i>r</i><sup>2</sup> = 0.72) and agricultural (<i>r</i><sup>2</sup> = 0.56) stream reach. The slopes of these regressions were in agreement with predicted assimilatory N-uptake based on additional metabolic data. Mean yearly assimilatory NO<sub>3</sub><sup>–</sup> uptake rates (U<sub>a</sub>) were 7.4 times higher in the agricultural stream reach (mean 78.3 mg N m<sup>–2</sup> d<sup>–1</sup>, max 270 mg N m<sup>–2</sup> d<sup>–1</sup>) than in the forest stream reach (mean 10.7 mg N m<sup>–2</sup> d<sup>–1</sup>, max 97.5 mg N m<sup>–2</sup> d<sup>–1</sup>). Nitrate uptake velocities (<i>V</i><sub>f,a</sub>) tended to decrease with increasing nitrate concentrations for periods with high light availability. Percentage daily assimilatory NO<sub>3</sub><sup>–</sup> uptake peaked at 47.4% of the daily NO<sub>3</sub><sup>–</sup> loading input to the stream network across the entire watershed, whereas the percentage yearly assimilatory NO<sub>3</sub><sup>–</sup> uptake was 9.0% of nitrogen loading to the stream network. This is a maximum because uptake can be revered by mineralization processes. The percentage yearly assimilatory NO<sub>3</sub><sup>–</sup> uptake was lower in the forest-dominated upstream subwatershed (4.8%) than in the lower agriculture dominated subwatershed (13.4%).