Incidence, Persistence and Effects of Soil Hydrophobicity Caused By Wildfire in Humid Hardwood Forests.

Wildfires can induce soil hydrophobicity, which in turn can increase soil erosion rates, accelerate nutrient losses, and degrade water quality. Most studies regarding the incidence and persistence of soil hydrophobicity after wildfire have primarily been conducted in semi-arid, coniferous forests, such as those of the western United States. It is not currently known if wildfires in humid hardwood forests, such as those commonly found in the southern Appalachian Mountains, induce water repellency in soil (i.e., hydrophobic conditions). It is also not well understood if wildfires in these regions alter the partitioning of rainfall into infiltration versus overland flow. In this study, we measured the incidence and persistence of soil hydrophobicity in two humid hardwood forests, one located in Mount Pleasant Wildlife Refuge, Virginia, and the other located in Chimney Rock State Park, North Carolina. In each location, sites were selected to represent unburned conditions as well as moderate and heavy burn intensities. Soil hydrophobicity was measured at multiple points in each site (n = 10) using the water drop penetration time (WDPT) method at 0 cm, 2 cm, and 5 cm depths. Soil water content and temperature were measured using a hand-held probe, and infiltration tests were conducted using a mini-disk tension infiltrometer. Hydrophobicity was commonly detected in the moderately and heavily burned sites, but was rarely found in the unburned sites. The strongest hydrophobicity (i.e., WDPT > 60 sec) was seen at the surface (0 cm) for Mountain Pleasant and in the shallow subsurface (2 cm) for Chimney Rock. Soil hydrophobicity persisted for more than ten months at each site, and was seen to vary for a given location depending on relative soil water content. Soil water contents in heavy and moderate burned sites were lower than unburned sites, which indicated either that hydrophobicity was preventing rewetting of burned soils, or else that those soils had lower water retention capacities post-fire. The infiltration tests revealed that burned soils had lower infiltration rates than unburned soils in the Mount Pleasant site, while infiltration rates in the Chimney Rock site did not significantly differ between burned and unburned sites. These different responses may be related to the depth of maximum soil hydrophobicity (i.e., 0 cm for Mount Pleasant; 2 cm for Chimney Rock). Specifically, the presence of a wettable surface ash layer, such as was seen in Chimney Rock, may increase the permeability of that soil relative to conditions where the upper soil surface is hydrophobic, such as was seen in Mount Pleasant. Overall, this study provides important insight into how wildfires in the southeastern region affect soil properties and hydrological processes.