Tracing atmospheric nitrate deposition in a complex semiarid ecosystem using delta17O.

Area of Science:

• Environmental Chemistry
• Isotope Geochemistry
• Atmospheric Science

Background:

• Nitrate is a key nutrient and pollutant.
• Tracing atmospheric nitrate deposition is crucial for understanding ecosystem impacts.
• Existing methods using oxygen isotopes (delta18O) have limitations.

Purpose of the Study:

• To utilize the large 17O anomaly in nitrate as a tracer for atmospheric deposition.
• To assess the contribution of atmospheric nitrate to a semiarid ecosystem in southern California.
• To compare the efficacy of the 17O anomaly tracer with standard delta18O techniques.

Main Methods:

• Measurement of the isotopic composition of nitrate in aerosols, fog, and precipitation.
• Quantification of the 17O anomaly using delta17O values.
• Application of the 17O anomaly to trace nitrate deposition in soil, stream, and groundwater.

Main Results:

• A significant 17O anomaly (delta17O values from 20 to 30 per mil) was observed in atmospheric nitrate samples.
• The 17O anomaly proved to be a conserved and robust tracer for atmospheric nitrate deposition.
• A substantial fraction of nitrate in the local ecosystem was identified as being of atmospheric origin.

Conclusions:

• The 17O anomaly in nitrate is a superior tracer for atmospheric deposition compared to delta18O.
• Atmospheric nitrate significantly contributes to the nitrogen load in southern California's semiarid ecosystems.
• Nitrate of atmospheric origin is exported from the system with minimal biological processing.