Changes in a Humic Cambisol heated (100–500 °C) under laboratory conditions: The significance of heating time

The influence of the temperature and its duration, that are among the most important components to determine the fire effects on soil properties, were studied in the 0–5 cm layer of a Humic Cambisol developed over siliceous conglomerates and under a shrub of Erica australis. Soil was sandy loam with 8.5 g 100 g− 1 of organic matter, pH of 4.7 and available P about 0.16 mg kg− 1. The water stability index (WSI) was 64.9, which was considered good. Differential thermal analysis (DTA) showed the presence of two peaks, one endothermic until 177 °C and other exothermic between 229 °C and 579 °C. Samples were heated at 100 °C, 200 °C and 500 °C during 5, 15, 30 and 60 min after reaching the selected temperature. To avoid a temperature gradient in the profile a sample layer of 2 cm depth was used. Soil heating to 100 °C at the four heating times and to 200 °C for 5 min did not produce significant changes in pH, organic matter, total N, C/N ratio, available P, extractable cations and WSI. Soils heated to 200 °C for 15, 30 and 60 min and 500 °C for 5 min, showed a significant increase in available P (from 0.9 to a maximum of 17.6 mg kg−1) and divalent cations (Ca2+ from 4.4 to 46.3 and Mg2+ from 1.5 to 8.7 mg kg− 1). For soils heated to 500 °C for 15, 30 and 60 min, a clear increase in soluble Na+ content (from 0.5 to 1.8 mg kg−1) and pH (from 5.5 to 8.1), in addition to a significant decrease in organic matter (from 8.7% to 2.4%) was detected. As a consequence an important decrease in WSI was found (from 64.9% to 54.3%).