Forest Fire Effects on Soil Color and Texture

Abstract

Wildland fires are common in the western USA. We collected burned and unburned soils from an oak woodland and four mixed conifer sites to determine the effect of fire on soil color and texture. Time since burning ranged from 22 d to 3 yr. Soil color and texture were most noticeably altered in severely burned soil under concentrated fuel (1-2% of the land surface), compared with nearby slightly or moderately burned areas. After severe burning, a 1- to 8-cm-thick reddened layer formed at all of the sites, which was redder in hue and had higher chromas and values than the unburned soils. Underlying this layer was a blackened layer with a thickness of 1 to 15 cm and lower Munsell values. Redder hues in the burned soils were apparently the result of Fe oxide transformations and higher values the result of the nearly complete removal of organic matter. Organic C content was also significantly reduced in the blackened layers, so the lower Munsell values were probably due to the charring of the organic matter that remained. The reddened soil layers had significantly less clay than either the unburned soils or blackened layers, which had clay contents that were not significantly different from each other. Sand-sized aggregates formed in the surface soils during burning at four of the five sites, altering the particle-size distribution and resulting in coarser textures due to a greater proportion of sand. Burning produced a finer texture at one site due to an increase in the silt fraction, resulting from the decomposition of kaolinized sand grains. Poorly crystalline aluminosilicates, extracted with boiling 0.5 M KOH, increased in the reddened layer at three of the sites and may contribute to the cementation of the sand-sized aggregates.