Seasonal nitrate uptake and denitrification potential in small headwater streams in the Willamette Valley, Oregon

Background/Question/Methods

Headwater streams can serve as important sources and sinks for nitrogen (N) for downstream receiving waters.  Prior research on N removal in small streams has largely focused on growing season conditions.  Here we examine the influence of headwater streams on annual watershed N export in the Willamette River valley, Oregon, USA.  Since N export is strongly seasonal in this region, we quantify nitrate removal during a range of flow and temperature conditions in summer, winter and spring.   We used a pulse addition of nitrate combined with a conservative tracer to measure nitrate uptake in intermittent and perennial streams (n=4 each) draining watersheds with varying proportions of forest and agriculture in the Calapooia River basin.  Laboratory assays of sediment denitrification potential were used to estimate the removal potentially attributable to denitrification.  Higher flows during winter conditions are expected to result in lower overall uptake, while spring conditions may have high removal due to warmer conditions, open canopies over the streams and higher nitrate concentrations than during summer. 

Results/Conclusions

Nitrate concentrations were low in all streams (4 - 231 mg NO₃-N L^-1) during the summer, and substantially higher in the winter (10-3,900 mg NO₃-N L^-1).  During the summer, ambient uptake lengths, rates, and velocities ranged from 60 - 740 m, 7.6 - 271.2 mg m^-2 min^-1, and 0.18 - 2.96 mm min^-1, respectively.  This translates to 75 - 99% of nitrate loaded at one location removed within 1 km downstream.  Sediment assays suggested nitrate availability limited denitrification during the summer, with half-saturation coefficients ranging from 499 to >10,000 mg NO₃-N L^-1.  Our initial results from summer conditions suggest that in-stream nitrate removal contributes to the low summer nitrate concentrations observed in these streams in agricultural systems.  The expansion of the stream network in winter and spring may contribute substantially to N losses from the watersheds through export and denitrification.