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

Background/Question/Methods

Between 10-20 % of yearly global N_₂O emissions are estimated to come from streams and rivers. Main sources of N_₂, NO and N_₂O 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 N_₂O along with NO₃^- and N_₂O mass flux, and stream discharge to quantify in-stream nitrogen source partitioning and transformation, and refine estimates for N_₂, NO and N_₂O 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 N_₂O produced by streams. As discharge (0.2 L/s – 99.5 L/s) and NO₃^- concentration (0.1 mg/L – 16 mg/L) increased from August to February along the stream of study so did the concentration of N_₂O (0.1 ppm – 6.5 ppm), leading to changes in mass flux of N_₂, NO and N_₂O. Isotopic analysis of NO₃^- indicates downstream enrichment of ¹⁵N (¹⁵N = 4.4 ‰ to ¹⁵N = 7.9 ‰). A persistent yearly enrichment of ¹⁵N-NO₃^- indicates 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.