Changes in Soil Mineralogy and Texture Caused by Slash‐and‐Burn Fires in Sumatra, Indonesia

We investigated the effect of fire intensity from slash‐and‐burn agriculture on the mineralogy of Oxisols in the Sepunggur area, Jambi Province, Sumatra, Indonesia, in both field and laboratory experiments. Samples were collected from two depths (0–5 and 5–15 cm) at locations exposed to 100, 300, 600, and >600°C surface temperatures during the burns. Soils under forest and slashed vegetation were collected as controls. The pre‐burn soil mineralogy was dominated by kaolinite, gibbsite, anatase, and goethite. Changes in soil properties with burning were most pronounced in the 0‐ to 5‐cm layer. Burning the topsoil led to coarser textures, especially at temperatures exceeding 600°C. Heat reduced the gibbsite and kaolinite concentrations and converted goethite into ultra‐fine maghemite, thus increasing the magnetic susceptibility of the samples. The conversion of goethite did not take place until water in the samples had vaporized. Addition of organic matter to soil with a low organic C content before heating increased the magnetic susceptibility, indicating that organic matter was necessary (and limiting) for the complete conversion of goethite. Coarse‐grained magnetite particles were present prior to and after the burning and, therefore, were not pyrogenic. Magnetic susceptibility measurements were highly discriminatory among heat treatments, whereas x‐ray diffraction (XRD) was much less sensitive to fire‐induced changes in mineralogy. Our research showed that severe burning had drastic effects on soil mineralogy, but changes should also be expected at lower fire intensities. Further research is needed to determine how important these changes in soil mineralogy are for nutrient availability in the growing season after the burn.