Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 9/8/2006
Publication Date: 11/1/2006
Citation: Moravec, B., Keller, C.K., Smith, J.L., Evans, D. 2006. Going After Nitrate: Utilizing Oxygen Isotopes to Identify Nitrogen Cycling Mechanisms in Soils. American Geophysical Union.
Technical Abstract: Nitrate pollution as a result of over utilization of fertilizers has recently received considerable interest because of its harmful effects on the environment. Studies have been carried out to identify sources of surface/ground-water nitrate through the use of dual isotope analysis, d15N and d18O. However, mechanisms of N cycling in soils, before discharge to surface waters as NO3, are not well understood. Soil and surface waters from nested catchments near Pullman, WA were sampled bi-monthly from 2001 and analyzed for d18O of water and NO3. The d18O of precipitation was also measured. Nitrate concentrations in soil and surface waters were shown to increase during heavy precipitation months (November–May) up to 35-80 mg/L, and decrease during periods of low precipitation (June–October) as low as 0-5 mg/L. Temporal patterns in d18O of water generally exhibited depletion during winter and spring, and suggested a mean residence time of 4-7 months for precipitation to reach surface drainage. In the present study, d18O values of soil/surface water and precipitation will be compared to d18O of nitrate in soil/surface waters. The latter will be measured using the denitrifier method. Expected results will likely fall within two possible scenarios: 1) d18O of nitrate will exhibit a d18O signature from fall soil moisture, suggesting that nitrate is formed shortly after application, then gets flushed rapidly through the soil. 2) d18O of NO3 will follow the average d18O for soil/surface waters and will not show temporal variability, suggesting that nitrate is cycled by plants and mixed within the soil profile before being transported to lower soil depths or surface drainage.