Relationships between organic matter, hydrophobic substances, and soil water repellency after wildfire in the Blue Ridge Mountains

Wildfires can induce soil water repellency, which influences biophysical processes such as infiltration, overland flow, soil erosion and nutrient losses. Previous studies examining causes and effects of post-wildfire soil water repellency have been conducted primarily in oak and conifer forests. By contrast, the occurrence and origin of soil water repellency in humid hardwood forests, such as those common in the eastern United States, are still not well understood. Our study objectives were therefore to identify 1) if and for how long wildfires in humid hardwood forests induce water repellency, and 2) how organic carbon content and hydrophobic functional groups explain actual and potential (i.e., at air-dry conditions) soil water repellency. Working in two forests, one located in Mount Pleasant Wildlife Refuge, Virginia, and the other in Chimney Rock State Park, North Carolina, United States, we selected sites that burned during wildfires in late 2016, along with unburned controls. We then repeatedly measured actual soil water repellency in the field using the water drop penetration time (WDPT) method, and collected unconsolidated samples to measure potential water repellency, total organic carbon content using C/N analyzer, and organic hydrophobic functional groups using FTIR. We also produced a water repellent index that quantifies the influence of total organic carbon content and hydrophobic functional groups on soil water repellency. The field results showed that the wildfires caused soil water repellency in both locations. Soil water repellency persisted one year in Mount Pleasant and decreased in Chimney Rock; the depth of observed water repellency changed through time after wildfire, moving deeper in the profile and possibly affecting the surrounding unburned soils. Total organic carbon content and hydrophobic functional groups together explained > 50% of variance in potential soil water repellency (except the subsoil in B horizon), but were not correlated with soil water repellency in the field, likely due to the influ ence of soil water content. This study provides new insight into the influence of soil organic matter content and characteristics on soil water repellency.