Effects of Fire Severity on Nitrate Mobilization in Watersheds Subject to Chronic Atmospheric Deposition

Abstract

Severe fires in chaparral watersheds subject to air pollution from metropolitan Los Angeles mobilized accumulated nitrogen and caused streamwater to be polluted with nitrate at concentrations exceeding the Federal Water Quality Standard. Streamwater NO3- concentrations were elevated during peak flows, the largest of which was a debris flow that transported NO3- at concentrations as high as 1.12 mequiv/L. Annual NO3- loss from severely burned watersheds, averaging 1.2 kequiv/ha, was 40 times greater than that from areas that remained unburned. Fires of moderate intensity produced a more subdued response in stream discharge and soil nitrification and less than one-seventh the NO3- loss observed after severe burning. We infer that the combination of atmospheric deposition with severe wildfires provides a strong and recurrent source of nitrate that could contribute to existing groundwater pollution in parts of eastern Los Angeles County. Moderating the fire regime by prescribed burning could provide substantial mitigation. © 1994, American Chemical Society. All rights reserved.