Pathways for Pathways for nitrate release nitrate release from an alpine from an alpine

watershed: watershed: Determination Determination

using using 1515 N and N and 1818 O O

Donald H. CampbellDonald H. Campbell

Carol Kendall, Cecily C. Y. Chang, Carol Kendall, Cecily C. Y. Chang, Steven R. SilvaSteven R. Silva

Kathy A. TonnessenKathy A. Tonnessen

Study: ObjectiveStudy: Objective

1)1) Determine sources of nitrate in Determine sources of nitrate in precipitation, groundwater, and precipitation, groundwater, and surface watersurface water

2)2) Follow the fluxes and Follow the fluxes and transformations of nitrogen transformations of nitrogen through the hydrologic pathwaysthrough the hydrologic pathways

3)3) Link plot-scale measurements of Link plot-scale measurements of ecosystem processes to catchment-ecosystem processes to catchment-scale flux measurementsscale flux measurements

15N & 15N & 18O Isotopes 18O Isotopes 15N is useful to determine nitrate 15N is useful to determine nitrate

concentration in areas affected by concentration in areas affected by agriculture, but not in undisturbed agriculture, but not in undisturbed watershedswatersheds

- - 15N values for atmospheric 15N values for atmospheric deposition and microbial processes have deposition and microbial processes have similar similar 15N 15N (NO3)(NO3) values that over lap values that over lap

18O18O(NO3) (NO3) can be used to help separate can be used to help separate atmospheric sources of nitrate from atmospheric sources of nitrate from microbialmicrobial

Nitrate Double-Isotope Nitrate Double-Isotope TechniqueTechnique

Mircobial nitrification derives two Mircobial nitrification derives two oxygen’s from water molecules and one oxygen’s from water molecules and one from atmosphericfrom atmospheric

Knowing this, and Knowing this, and 18O of water and 18O of water and O2, you can calculate 18Oof microbial O2, you can calculate 18Oof microbial nitratenitrate

18O18O(NO3)(NO3)= 2/3 = 2/3 18O 18O(H20)(H20) + 1/3 + 1/3 18O18O(O2)(O2)

Using this model and the normal range Using this model and the normal range of values for of values for 18O 18O(H2O) (H2O) (-25 to+ 4%) and (-25 to+ 4%) and 18O of soil O2 gas (about +23%), then 18O of soil O2 gas (about +23%), then you get a range of -10 to +10% 18Oyou get a range of -10 to +10% 18O(NO3)(NO3)

Nitrate Double-Isotope Nitrate Double-Isotope TechniqueTechnique

Model is built on four assumptions,Model is built on four assumptions,

1)1) Oxygen to water proportions are the Oxygen to water proportions are the same in soils as they are in lab same in soils as they are in lab culturescultures

2)2) No fractionationNo fractionation

3)3) 18O of water used by microbes is the 18O of water used by microbes is the same water that is being sampledsame water that is being sampled

4)4) 18O of O2 used by the microbes is the 18O of O2 used by the microbes is the same as the atmospheric O2same as the atmospheric O2

Nitrate Double-Isotope Nitrate Double-Isotope TechniqueTechnique

Problem is that under natural conditions Problem is that under natural conditions some of the assumptions may be violatedsome of the assumptions may be violated

Several studies have shown higher Several studies have shown higher 18O18O(NO3) (NO3) values by as much as 5% greater values by as much as 5% greater than the theoretically calculated value of than the theoretically calculated value of +10%+10%

Possibly due to fractionation of O2 during Possibly due to fractionation of O2 during respiration and a fluctuating ratio of respiration and a fluctuating ratio of oxygens derived from water, versus those oxygens derived from water, versus those derived from O2 gas derived from O2 gas

For the study a value of 0 to +15% was For the study a value of 0 to +15% was usedused

SiteSite /Sampling/Sampling

In Loch Vale watershed, Rocky Mtn. In Loch Vale watershed, Rocky Mtn. National ParkNational Park

Two main sub-basins, Icy Brook and Two main sub-basins, Icy Brook and Andrews Creek were sampledAndrews Creek were sampled

Two springs emerging from Talus Two springs emerging from Talus deposits were also sampleddeposits were also sampled

Results: SnowpackResults: Snowpack

Earliest snowmelt samples had Earliest snowmelt samples had isotopic nitrate compositions similar isotopic nitrate compositions similar to the winter snowpack, followed by to the winter snowpack, followed by decreasing decreasing O18 and O18 and N15 values N15 values

15N decreased from +55 to 65% in 15N decreased from +55 to 65% in winter down to +40% for spring winter down to +40% for spring

18O values changed from +2.2% in 18O values changed from +2.2% in winter to +.5% for springwinter to +.5% for spring

Results: Stream WaterResults: Stream Water Both streams Icy Brook and Andrews Creek Both streams Icy Brook and Andrews Creek

had had 15N 15N(NO3)(NO3) values from -1 to +2% and values from -1 to +2% and 18O18O(NO3) (NO3) values that ranged from +10 to values that ranged from +10 to +25%+25%

A sample collected early April 1996, from A sample collected early April 1996, from Andrews Creek, had nearly identical Andrews Creek, had nearly identical 18O18O(NO3) (NO3) values compared to the atmospheric values compared to the atmospheric deposition valuesdeposition values

*A similar event was observed during early *A similar event was observed during early snowmelt in 1994snowmelt in 1994

Results: Talus SpringsResults: Talus Springs

Nitrate concentrations observed ranged Nitrate concentrations observed ranged from 24 to 64 ueq/L and from 24 to 64 ueq/L and 18O 18O(NO3)(NO3) values were between +10 to +27%values were between +10 to +27%

The talus springs had high nitrate The talus springs had high nitrate concentrations and a seasonal pattern concentrations and a seasonal pattern in in 18O 18O(NO3) (NO3) that was observed in the that was observed in the streamsstreams

Important sources of nitrate later in the Important sources of nitrate later in the summersummer

Seasonal PatternSeasonal Pattern Nitrate Concentrations in the stream water, Nitrate Concentrations in the stream water,

peaked in late May, decreased throughout peaked in late May, decreased throughout the summer, and then started to rise again in the summer, and then started to rise again in the Fallthe Fall

The seasonal variation is believed to be The seasonal variation is believed to be caused from differences in storm patterns caused from differences in storm patterns and varying sources of nitrogen oxide in the and varying sources of nitrogen oxide in the front rangefront range

Despite seasonal variation values, from 1995-Despite seasonal variation values, from 1995-1997 were constant for 1997 were constant for 18O and 18O and 15N 15N values of nitrate, with only a 1% difference in values of nitrate, with only a 1% difference in the range for the range for 15N, and 5% for 15N, and 5% for 18O. 18O.

Fluxes and Residence Fluxes and Residence TimeTime

A relatively small seasonal variance A relatively small seasonal variance in in 18O 18O(H20) (H20) was seen, suggesting a was seen, suggesting a long residence time and large long residence time and large reservoir of waterreservoir of water

Residence time was thought to be Residence time was thought to be almost a year for Andrews Creek, almost a year for Andrews Creek, allowing allowing

ResultsResults 18O18O(NO3)(NO3) values for Talus springs and streams values for Talus springs and streams

indicated that most of the nitrate had undergone indicated that most of the nitrate had undergone nitrification and was not directly from atmospheric nitrification and was not directly from atmospheric depositiondeposition

Despite the apparent lack of soil, microbial cycling Despite the apparent lack of soil, microbial cycling controls the supply of nitrate in the Loch Vale controls the supply of nitrate in the Loch Vale watershedwatershed

The fluxes of nitrate being exported from the The fluxes of nitrate being exported from the system are a function of the hydrologic variables system are a function of the hydrologic variables (flow paths, residence time, and reservoir sizes)(flow paths, residence time, and reservoir sizes)

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