Nitrate uptake in an agricultural stream estimated from high-frequency, in-situ sensors

Real-time, continuous, in situ water quality sensors were deployed on a fourth-order Iowa (U.S.) stream draining an agricultural watershed to evaluate key in-stream processes affecting concentrations of nitrate during a 24-day late summer (Aug-Sep) period. Overall, nitrate-nitrogen (NO₃-N) concentrations declined 0.11 mg L^-1 km^-1, or about 1.9% km^-1 and 35% in total across 18 km. We also calculated stream metabolic rates using in situ dissolved oxygen data and determined stream biotic N demand to be 108-117 mg m^-2 day^-1. From this, we estimate that 11% of the NO₃-N concentration decline measured between two in-situ sensors separated by 2 km was a result of biotic NO₃-N demand, while groundwater NO₃-N data and estimates of groundwater flow contributions indicate that dilution was responsible for 53%. Because the concentration decline extends linearly across the entire 18 km of stream length, these processes seem consistent throughout the basin downstream of the most upstream sensor site. The nitrate-dissolved oxygen relationship between the two sites separated by 2 km, calculations of biotic NO₃-N demand, and diurnal variations in NO₃-N concentration all indicate that denitrification by anaerobes is removing less NO₃-N than that assimilated by aquatic organisms unable to fix nitrogen for their life processes, and thus the large majority of the NO₃-N entering this stream is not retained or removed, but rather transported downstream.