Effects of fire and ash on soil water retention

Despite the pronounced effect of fire on soil hydrological systems, information on the direct effect of fire on soil water retention characteristics is limited and contradictory. To increase understanding in this area, the effect of fire on soil water retention was evaluated using laboratory burning and heating experiments. In addition, ash-infiltration and ash-incorporation experiments were performed to evaluate the effect of ash on soil water retention. While heating soil to 200 °C and below did not change soil properties, burning and heating to 300 °C and above increased bulk density, clay and silt content, and decreased organic matter and sand content. Burning and heating above 200 °C decreased the amount of water stored at the nine tensions considered, although the effect on soil water retention did not always increase with increasing temperature. Changes were largest for low tensions, i.e. between saturation and field capacity (10 kPa). Heating to 200 °C decreased the amount of plant available water, but despite reducing the amount of water stored at evaluated tensions, burning and heating to 300 °C and above increased the amount of plant available water. This may be caused by more complete combustion of organic matter at the higher temperatures and the production of ash. Direct incorporation of ash into soils did not alter soil texture but increased water retention from saturation to 310 kPa tension. Ash infiltration experiments interestingly had a similar effect, despite the fact that very little ash washed into the samples. Results from these experiments contribute to understanding post-fire changes in hydrological and erosion processes. ► Burning and heating above 300 °C changed soil texture, bulk density and OM content. ► Burning and heating decreased the amount of water held at most tensions. ► Effects were most pronounced at low tensions. ► Effects did not simply increase with increasing temperature. ► Ash incorporation and infiltration increased soil water retention.