1 / 13

Pathways for nitrate release from an alpine watershed: Determination using  15 N and  18 O

This study aims to determine the sources of nitrate in precipitation, groundwater, and surface water in an alpine watershed. Isotope analysis of nitrogen and oxygen is used to track the fluxes and transformations of nitrogen through hydrologic pathways. The study also links plot-scale ecosystem measurements to catchment-scale flux measurements.

sappt
Download Presentation

Pathways for nitrate release from an alpine watershed: Determination using  15 N and  18 O

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Pathways for nitrate release from an alpine watershed: Determination using 15 N and 18 O Donald H. Campbell Carol Kendall, Cecily C. Y. Chang, Steven R. Silva Kathy A. Tonnessen

  2. Study: Objective • Determine sources of nitrate in precipitation, groundwater, and surface water • Follow the fluxes and transformations of nitrogen through the hydrologic pathways • Link plot-scale measurements of ecosystem processes to catchment-scale flux measurements

  3.  15N &  18O Isotopes •  15N is useful to determine nitrate concentration in areas affected by agriculture, but not in undisturbed watersheds -  15N values for atmospheric deposition and microbial processes have similar  15N (NO3) values that over lap •  18O(NO3) can be used to help separate atmospheric sources of nitrate from microbial

  4. Nitrate Double-Isotope Technique • Mircobial nitrification derives two oxygen’s from water molecules and one from atmospheric • Knowing this, and  18O of water and O2, you can calculate 18Oof microbial nitrate  18O(NO3)= 2/3  18O(H20) + 1/3  18O(O2) • Using this model and the normal range of values for  18O(H2O) (-25 to+ 4%) and  18O of soil O2 gas (about +23%), then you get a range of -10 to +10% 18O(NO3)

  5. Nitrate Double-Isotope Technique • Model is built on four assumptions, • Oxygen to water proportions are the same in soils as they are in lab cultures • No fractionation • 18O of water used by microbes is the same water that is being sampled • 18O of O2 used by the microbes is the same as the atmospheric O2

  6. Nitrate Double-Isotope Technique • Problem is that under natural conditions some of the assumptions may be violated • Several studies have shown higher  18O(NO3) values by as much as 5% greater than the theoretically calculated value of +10% • Possibly due to fractionation of O2 during respiration and a fluctuating ratio of oxygens derived from water, versus those derived from O2 gas • For the study a value of 0 to +15% was used

  7. Site /Sampling • In Loch Vale watershed, Rocky Mtn. National Park • Two main sub-basins, Icy Brook and Andrews Creek were sampled • Two springs emerging from Talus deposits were also sampled

  8. Results: Snowpack • Earliest snowmelt samples had isotopic nitrate compositions similar to the winter snowpack, followed by decreasing  O18 and  N15 values •  15N decreased from +55 to 65% in winter down to +40% for spring •  18O values changed from +2.2% in winter to +.5% for spring

  9. Results: Stream Water • Both streams Icy Brook and Andrews Creek had  15N(NO3) values from -1 to +2% and  18O(NO3) values that ranged from +10 to +25% • A sample collected early April 1996, from Andrews Creek, had nearly identical  18O(NO3) values compared to the atmospheric deposition values *A similar event was observed during early snowmelt in 1994

  10. Results: Talus Springs • Nitrate concentrations observed ranged from 24 to 64 ueq/L and  18O(NO3) values were between +10 to +27% • The talus springs had high nitrate concentrations and a seasonal pattern in  18O(NO3) that was observed in the streams • Important sources of nitrate later in the summer

  11. Seasonal Pattern • Nitrate Concentrations in the stream water, peaked in late May, decreased throughout the summer, and then started to rise again in the Fall • The seasonal variation is believed to be caused from differences in storm patterns and varying sources of nitrogen oxide in the front range • Despite seasonal variation values, from 1995-1997 were constant for  18O and  15N values of nitrate, with only a 1% difference in the range for  15N, and 5% for  18O.

  12. Fluxes and Residence Time • A relatively small seasonal variance in  18O(H20) was seen, suggesting a long residence time and large reservoir of water • Residence time was thought to be almost a year for Andrews Creek, allowing

  13. Results •  18O(NO3) values for Talus springs and streams indicated that most of the nitrate had undergone nitrification and was not directly from atmospheric deposition • Despite the apparent lack of soil, microbial cycling controls the supply of nitrate in the Loch Vale watershed • The fluxes of nitrate being exported from the system are a function of the hydrologic variables (flow paths, residence time, and reservoir sizes) • Both biogeochemical and hydrologic processes control the export of nitrate in alpine streams and springs.

More Related