COS 126-1 - Improving estimates of in-stream nitrogen transformations by applying spatiotemporal variability in a semi-arid agricultural stream

Thursday, August 9, 2012: 8:00 AM
F150, Oregon Convention Center
Leif G. Moon-Nielsen and Cailin Huyck Orr, School of the Environment, Washington State University, Pullman, WA
Background/Question/Methods

Between 10-20 % of yearly global NO emissions are estimated to come from streams and rivers. Main sources of N, NO and NO in streams and rivers are produced by dissimilatory microbial nitrogen transformation (nitrification and denitrification). Current estimates of these transformations in freshwater aquatic systems have been based upon measurements at one time of the year and in one location in a stream or river. To improve current estimates we investigated nitrogen transformations at multiple locations occurring in a set of nested catchments ranging in size from 600 Ha to 5000 Ha during 1 yr. in a semi-arid agricultural stream in eastern Washington State, USA. Questions asked were: 1) At what rate are streams processing labile nitrogen and transforming it to gaseous forms of nitrogen? 2) How do these processing rates fluctuate through the year? 3) To what extend does seasonal surface hydrology affect nitrogen transformation rates? Here we utilize stable isotope analysis to determine the 15N and 18O ratio of in-stream NO3- and NO along with NO3- and NO mass flux, and stream discharge to quantify in-stream nitrogen source partitioning and transformation, and refine estimates for N, NO and NO production.

Results/Conclusions

In eastern Washington State, annual stream discharge peaks between the months of February and April. Peak discharge is caused by snow melt and rain-on-snow events. NO3- concentration and discharge are strongly coupled, and likely influence the amount of NO produced by streams. As discharge (0.2 L/s – 99.5 L/s) and NO3concentration (0.1 mg/L – 16 mg/L) increased from August to February along the stream of study so did the concentration of NO (0.1 ppm – 6.5 ppm), leading to changes in mass flux of N, NO and NO. Isotopic analysis of NO3indicates downstream enrichment of 15N (15N = 4.4 ‰ to 15N = 7.9 ‰). A persistent yearly enrichment of 15N-NO3indicates the dominance of denitrification in aquatic nitrogen transformation in this area. Results indicate hydrologic regime as key to understanding seasonal variability in nitrogen transformation and therefore improving previous estimates of stream nitrogen transformation rates.