EFFECTS OF FIRE SEVERITY ON NITRATE MOBILIZATION IN WATERSHEDS SUBJECT TO CHRONIC ATMOSPHERIC DEPOSITION

Severe fires in chaparral watersheds subject to air pollution from met ropolitan Los Angeles mobilized accumulated nitrogen and caused stream water to be polluted with nitrate at concentrations exceeding the Fede ral Water Quality Standard. Streamwater NO3-concentrations were elevat ed during peak flows, the largest of which was a debris flow that tran sported NO3- at concentrations as high as 1.12 mequiv/L. Annual NO3- l oss from severely burned watersheds, averaging 1.2 kequiv/ha,was 40 ti mes greater than that from areas that remained unburned. Fires of mode rate intensity produced a more subdued response in stream discharge an d soil nitrification and less than one-seventh the NO3- loss observed after Severe burning. We infer that the combination of atmospheric dep osition with severe wildfires provides a strong and recurrent source o f nitrate that could contribute to existing groundwater pollution in p arts of eastern Los Angeles County. Moderating the fire regime by pres cribed burning could provide substantial mitigation.