Soil color as an indicator of slash-and-burn fire severity and soil fertility in Sumatra, Indonesia

Fire is widely used as a tool for converting forest to agricultural land in many developing countries, and correlations are thought to exist between fire severity, burned soil color, and soil fertility. To test this hypothesis, field experiments were conducted in Sepunggur, Jambi Province, Sumatra, Indonesia. Field burning a slashed 12- to 15-yr-old secondary forest caused Munsell values and chromas to decrease and hues to become yellower with increasing heat severity, especially in the top 5 cm of the soil. However, at peak surface temperatures >600 degrees C, soil C was mostly depleted and the soil matrix was reddened. Laboratory studies showed similar results with static heating. Moreover, color changes were highly dependent on the duration of exposure at a given temperature. Fire induced the formation of aggregates with exteriors that had lower values and chromas and slightly redder hues than the interiors. Laboratory removal of organic matter from burned samples by chemical oxidation did not alter the color. Soil exchangeable Ca, Mg, and K increased with fire severity, while exchangeable acidity and Al decreased 2 wk after the burn. Soil C and N were reduced at high burn severity only. Phosphorus showed an increase in availability at low to medium fire severity and a decrease in availability at the most intense burn levels. Colors of burned areas in the field did not change significantly during the 12 wk following the burn. However, within It wk following the field burn exchangeable Ca had decreased to preburn levels and in saturation had increased markedly. Using postburn color measurements to predict the spatial patterns in soil fertility was limited by the fact that fertility changed rapidly following the burn, whereas color parameters did not.