In-stream net uptake regulates inorganic nitrogen export from catchments under base flow conditions

We aimed to investigate the temporal variation of in-stream net dissolved inorganic nitrogen (DIN) areal uptake rates (U-DIN, in mu g N m(-2) min(-1)) and its implications on regulating catchment N export, under base flow conditions. To do so, we estimated U-DIN from longitudinal profiles of ambient DIN concentration (nitrate + ammonium) in two streams on a monthly basis during two hydrological years (n = 45). We found that in-stream DIN uptake and release did not offset each other (U-DIN not equal 0) in half of the dates, and that U-DIN > 0 occurred mostly in autumn. Based on these reach-scale uptake rates, we performed empirical calculations and model simulations to assess the potential of stream network DIN retention to regulate DIN export from catchments on an annual scale. The empirical approach consisted in up-scaling U-DIN by means of a dynamic stream network analysis that considered temporal and spatial variation of U-DIN. The modeling approach consisted in applying different scenarios with the INCA model based on the natural range of empirical U-DIN values. Our results showed that the contribution of stream network DIN retention to catchment DIN export increased when calculations accounted for the temporal variation of U-DIN. Both approaches suggested that stream network DIN retention can significantly reduce DIN export from headwater catchments under base flow conditions (from 4% to 38%).