Effects of fire on the physicochemical properties of soil in a slash-and-burn agriculture

• Fire in slash-and-burn reached temperatures as high as those in wildfire areas.
• Chemical changes only occurred to a depth of 2.5 cm in the topsoil.
• Aggregate stability tended to decrease only to a depth of 2.5 cm in the topsoil.
• Fire had effect only on the chemical properties of the soil.
• Fire did not destroy the water repellency of the topsoil.

Fire is an important physical agent that influences several environmental processes. It is used in slash-and-burn agriculture as a tool for land management, where the heat generated during burning can change physical, chemical and biological soil properties. Therefore, the fallow period plays a key role in both increasing sustainability and reducing soil degradation in slash-and-burn agricultural systems, as it allows the physicochemical and biological properties of the soil to be stored. The objective of this study was to evaluate the immediate effect of fire on the physicochemical properties of the topsoil (0–5 cm) for a slash-and-burn agricultural system in a subtropical environment following a 7–8 year fallow period during which 6.2 kg m− 2 of biomass had accumulated. A 0.8-ha plot was slashed, dried, and burned according to the local slash-and-burn system used for cropping maize and black beans. Soil temperatures were measured at depths of 0 to 5 cm and > 500 °C were recorded at the soil surface and < 100 °C at 5 cm depth, probably due to the evaporative cooling effect of the relative moist soil. Physicochemical changes to the soil were observed up to a depth of 2.5 cm. Changes in the chemical properties of the soil were more evident than changes in the physical properties. Although the soil surface was heated to > 300 °C, the temperature at which any soil water repellency is typically destroyed; repellency levels remained high (~ 250 s, WDPT) after the fire. The outcomes suggest that even relatively high fuel loads and densities in this tropical slash-and-burn system do not necessarily have a detrimental effect on soil physical parameters.