Prescription Fire and Anion Retention in Tahoe Forest Soils

Abstract

Prescribed burning is a management option to reduce fire hazard in the Lake Tahoe Basin. However, subsequent nutrient loading to the lake is of major concern. The effect of prescribed burning and residual ash on soil chemical properties and anion concentrations was studied in both the field (Tahoe National Forest) and laboratory. Saw- tooth and Marlene field sites were chosen on the basis of the dominant parent materials found in the Tahoe Basin: andesite and decomposed granite, respectively. Field studies consisted of prebum, postbum, and post-snowrnelt soil analyses. Laboratory experiments consisted of var- iable ash to soil mixtures followed by membrane extraction. Measured burn intensities varied within and between study sites with a mean soil temperature of 190 °C ± 168 °C at Sawtootls (andesite) and 402 °C.1 198 °C at Marlene (decomposed granite). Respective burn intensities were not correlated to any ion concentration at either site. Soil samples collected posthurn and post-snowmelt indicated that burning tended to increase soil p1-I, extractable calcium (Ca), and water-soluble sulfate (SO42 ). although not always statistically significant. No consistent trend was observed for orthophosphate. Laboratory additions of ash to both unburned soils resulted in a substantial increase in but pH and SO42 ; conversely, extractable orthophosphate decreased. The increase in SO4 2 - far exceeded the content of the ash and thus is hypothesized to be caused by pH-induced desorption from the unburned soil. Results were similar for both parent materials, indicating that nutrient retention may be controlled by soil p1-I resulting from ash incorporation.