Tracing Atmospheric Nitrate Deposition in a Complex Semiarid Ecosystem Using Δ¹⁷O

The isotopic composition of nitrate collected from aerosols, fog, and precipitation was measured and found to have a large ¹⁷O anomaly with Δ¹⁷O values ranging from 20‰p to 30‰ (Δ¹⁷O = δ¹⁷O - 0.52(δ¹⁸O)). This ¹⁷O anomaly was used to trace atmospheric deposition of nitrate to a semiarid ecosystem in southern California. We demonstrate that the Δ¹⁷O signal is a conserved tracer of atmospheric nitrate deposition and is a more robust indicator of N deposition relative to standard δ¹⁸O techniques. The data indicate that a substantial portion of nitrate found in the local soil, stream, and groundwater is of atmospheric origin and does not undergo biologic processing before being exported from the system.