Forest fire effects on soil morphology and mineralogy.

摘要

Wildland fires are a common occurrence throughout the western United States. The objectives of this study were to determine the effect of fire on soil color, texture, iron oxides, and phyllosilicates. Burned and unburned soils were collected from an oak woodland and four mixed conifer sites in California. Time since burning ranged from 22 days to 3 years. Soil morphology and mineralogy were most noticeably altered in the severely burned soil under concentrated fuel (1-2% of land surfaces), compared to nearby slightly or moderately burned areas. The soil temperatures during severe burning ranged from 400{dollar}spcirc{dollar}C to over 850{dollar}spcirc{dollar}C and were estimated by comparing the thermogravimetric weight loss curves of the unburned and burned soils.; During severe burning, a 1- to 8-cm-thick surface soil layer formed which had redder hue and higher Munsell value and chroma than the unburned soil. Underlying this was a blackened soil layer with a thickness of 1 to 15 cm and lower Munsell values. Light colored wood-ash, containing CaCO{dollar}sb3{dollar}, remained at the surface of three of the sites and increased the pH of the burned soil significantly. Redder hues in the severely burned soils were probably due to increased hematite, and possibly maghemite, concentrations, or to increased crystallinity of preexisting hematite. Removal of organic matter in the reddened soils resulted in higher Munsell values.; The burned, reddened soils had significantly less clay than either the unburned or blackened soils. In soils at four of the sites, the sand fraction increased due to the formation of sand-sized aggregates during burning. These aggregates may have been cemented by the elevated levels of poorly crystalline alumino-silicates in the severely burned surface soils at three of the sites. At one site, burning produced a finer texture because the decomposition of kaolinized sand grains increased the silt content.; This study has demonstrated the decomposition of kaolin in the severely burned surface soils, as well as the dehydroxylation and collapse of chlorite, randomly interstratified chlorite/vermiculite, hydroxy-interlayered-vermiculite, vermiculite, and biotite. The morphological and mineralogical changes due to burning observed in this study were evident three years after a fire.