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

Environ Monit Assess

IIHR-Hydroscience and Engineering, University of Iowa, 306 C. Maxwell Stanley Hydraulics Laboratory, Iowa City, IA, 52242, USA.

Published: March 2018

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 (NO-N) concentrations declined 0.11 mg L km, or about 1.9% km 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 day. From this, we estimate that 11% of the NO-N concentration decline measured between two in-situ sensors separated by 2 km was a result of biotic NO-N demand, while groundwater NO-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 NO-N demand, and diurnal variations in NO-N concentration all indicate that denitrification by anaerobes is removing less NO-N than that assimilated by aquatic organisms unable to fix nitrogen for their life processes, and thus the large majority of the NO-N entering this stream is not retained or removed, but rather transported downstream.

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Source
http://dx.doi.org/10.1007/s10661-018-6599-1DOI Listing

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