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

Study: Objective 1) Determine sources of nitrate in precipitation, groundwater, and surface water 2) Follow the fluxes and transformations of nitrogen through the hydrologic pathways 3) Link plot-scale measurements of ecosystem processes to catchment-scale flux measurements

 15N &  18O Isotopes  15N is useful to determine nitrate concentration in areas affected by agriculture, but not in undisturbed watersheds  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  18O (NO3) can be used to help separate atmospheric sources of nitrate from microbial

Nitrate Double-Isotope Technique Mircobial nitrification derives two oxygen’s from water molecules and one from atmospheric 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 Knowing this, and  18O of water and O2, you can calculate 18Oof microbial nitrate  18O (NO3) = 2/3  18O (H20) + 1/3  18O (O2)  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) 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)

Nitrate Double-Isotope Technique Model is built on four assumptions, Model is built on four assumptions, 1) Oxygen to water proportions are the same in soils as they are in lab cultures 2) No fractionation 3) 18O of water used by microbes is the same water that is being sampled 4) 18O of O2 used by the microbes is the same as the atmospheric O2

Nitrate Double-Isotope Technique Problem is that under natural conditions some of the assumptions may be violated 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% 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 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 For the study a value of 0 to +15% was used

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

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

Results: Stream Water Both streams Icy Brook and Andrews Creek had  15N (NO3) values from -1 to +2% and 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 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

Results: Talus Springs Nitrate concentrations observed ranged from 24 to 64 ueq/L and  18O (NO3) values were between +10 to +27% 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 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 Important sources of nitrate later in the summer

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 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 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 were constant for  18O and  15N values of nitrate, with only a 1% difference in the range for  15N, and 5% for  18O. Despite seasonal variation values, from were constant for  18O and  15N values of nitrate, with only a 1% difference in the range for  15N, and 5% for  18O.

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

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  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 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) 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. Both biogeochemical and hydrologic processes control the export of nitrate in alpine streams and springs.