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Stream Chemistry and Watershed Nutrient Economy Following Wildfire and Fertilization in Eastern Washington¹

ABSTRACT

During the first 3 years after a severe wildfire in 1970, maximum concentrations of nitrate-N (NO₃-N) in stream water increased from prefire levels of <0.016 to 0.$6 mg/liter on a burned, unfertilized watershed and to 0.54 and 1.47 mg/liter on two watersheds that were burned and fertilized. Maximum NO₃-N concentration in the stream from an unburned watershed was 0.066 mg/liter. Since NO₃-N levels in the stream from the burned-only watershed were similar to burned-fertilized watersheds, increased levels appear to be primarily a result of increased nitrification. Organic N concentrations in streamflow were nearly doubled during the second year after fire compared to prefire levels.

Concentrations of total phosphorus (T-P) in streams from one burned and two burned-fertilized watersheds were 1.5 to 3 times greater than from an unburned watershed. Combined concentrations of Ca, Mg, K, and Na in streams prior to fire ranged from 12.0 to 14.9 mg/liter. Concentrations declined to 7.4–10.5 mg/liter in streams from burned and burned-fertilized watersheds during the second year after fire (1972) because of dilution resulting from increased discharge and were still less in 1975 than prefire levels (8.4 to 12.7 mg/liter).

Average inputs of N, P, and the four cations during the 5 years of study were 1.23, 0, and 3.56 kg/ha per year. Yearly N input from precipitation was sufficient to balance solution losses in three of the five postfire years. Cation losses in solution greatly exceeded precipitation inputs in all years.

Relative to background conditions and levels observed in the control stream, the fire exerted protracted impacts on the chemistry of these streams. Results however, indicate that fire and fertilization exerted negligible effects on chemical water quality for municipal use.

Key Words: streams • forest fire effects • fertilization (plants) urea • ammonium sulfate • nutrients (plants)

¹ Contribution from USDA For. Serv., Pacific Northwest For. and Range Exp. Stn., Forest Hydrology Lab., Wenatchee, WA 98801.

² Principal Range Scientist, Principal Hydrologist, and Chemist, respectively.

Received for publication October 12, 1977.