Soil water repellency after wildfires in the Blue Ridge Mountains, United States

It is not well understood if wildfires induce soil water repellency in broadleaf deciduous forests, such as those endemic to the Blue Ridge Mountains of the eastern United States. In 2016, widespread wildfires provided an opportunity to study soil water repellency in this region. We selected sites in four locations with low to moderate burn severities, along with unburned controls. We estimated soil water repellency using water drop penetration time measurements from the surface (i.e. ash or organic) layer to ∼5 cm within the underlying mineral layer. Two months after the fires, water repellency was detected in all locations and was greater in more severely burned sites. One location had the greatest water repellency in surface ash (frequency of occurrence: 68-74%), whereas the other locations showed greatest repellency at the ash-mineral interface (40-96%). Unburned soils rarely showed repellency (0-18%). Burned soils also exhibited water repellency 1 year post fire. The study results suggest that combustion of non-resinous foliage within litter layers can cause water repellency in deciduous forests, meaning that this condition is not exclusive to coniferous and dryland forests. The duration of impact depends on fire severity, and may enhance overland flow and sediment transport in affected landscapes.