Mineralogical and Chemical Alterations of Soils Caused by Combusted Woody Fuels in California Wildfires.

During wildfires, logs and large limbs on the forest floor, and roots under ground, are often thoroughly combusted generating prolonged elevation of soil temperatures in excess of 500° C.  We have studied these burned soils in forests throughout California.  A common and visually obvious alteration is the change in soil color from the yellow brown characteristic of goethite to reddish colors imparted by maghemite or hematite.  Concomitant increases in magnetic susceptibility indicate that maghemite is an iron oxide phase produced by this thermal process.  Changes in silicate mineralogy include decomposition of kaolin and the dehydroxylation and collapse of vermiculite and chlorite.  The high temperatures generated underneath burning logs and limbs eliminate virtually all soil organic matter and appreciably alter the cation exchange capacity of the soils.  We have found that this severe alteration of soils extends as deep as 80 mm and affects 1-15% of the land surface after wildfires, depending on fuel loads and fire severity.  Over the long-term, the severe thermal alteration of soils caused by burning of coarse woody debris and roots increases mineral weathering and the heterogeneity of forest soil properties.