Fire effects on cation exchange capacity of California forest and woodland soils

• High soil temperatures under dense burning fuel can consume SOM and alter minerals.
• The CEC of severely burned soil can be up to 82% lower than the unburned soil.
• Loss of SOM and thermal alteration of minerals results in lower CEC.
• CEC loss affects only small volumes of soil, even in dense forest stands.

During natural and prescribed fires, combustion of heavy surface fuel loads can expose underlying soil to high temperatures causing thermal alteration of soil minerals and other changes in soil properties. We studied the effect of fire on soil cation exchange capacity (CEC) by comparing severely burned soils to nearby unburned soils at five forest and woodland sites in California, USA. Combusted logs and heavy fuel produced reddened zones of thermally altered soil underlain by blackened zones where heat exposure was lower. The cation exchange capacities of the reddened soils were 57 to 82% lower than the unburned soil while the blackened soils had 31 to 53% less CEC than unburned soil. The reddened soil experienced the highest burning temperatures resulting in the loss of organic carbon (OC), the conversion of goethite to maghemite or hematite, the destruction of kaolin, and dehydroxylation and permanent collapse of vermiculite and chlorite. Overall, most of the CEC loss was due to thermal destruction of soil organic matter (SOM), but this CEC will be restored in the post-fire ecosystem succession as biomass re-accumulates and rebuilds soil humus. The loss of CEC due to mineral alteration is longer lasting, though limited in spatial extent.