Natural severity of water repellency in pine forest soils from NW Spain and influence of wildfire severity on its persistence

Soil water repellency is a widespread property of Pinus pinaster forest soils in Galicia (NW Spain) that can be intensified by soil heating during fires. Fire-induced or -enhanced soil water repellency, in combination with reduced vegetation and litter cover, is widely considered to be one of the main causes of increased overland flow and accelerated soil erosion on recently burnt hill slopes.In this work, we assessed the natural severity of water repellency in forest soils of different texture under P. pinaster in Galicia (NW Spain) and we examined the specific effect of a forest fire in an area under granite substrate and P. pinaster plantation on the severity and persistence of soil water repellency. Because the burnt area included zones that were affected to a different extent by the fire, we focused on the effect of its severity on soil water repellency from surface level down to a depth of 40 cm immediately after the fire and 15 months later. Persistency in the soil water repellency was assessed with the water drop penetration time test (WDPT).Water repellency in the sampled soils was found to be extremely persistent in the surface layer (0–5 cm). Also, the soils of a coarser texture, exhibit more persistence repellency and at greater depths than those of a finer texture.Fire severity was found to influence the persistence of water repellency and the thickness of the hydrophobic layer in the burnt zones. Such persistence in the soil affected by low-severity fire not differed significantly from that in unburnt soil; also, repellency remained severe to extreme down to 10 cm. On the other hand, soil water repellency in areas subject to fire of medium–high severity was completely suppressed within the top 2 cm of soil, but increased with depth (extreme water repellency between 2 and 20 cm). Fifteen months after the fire, both burnt and unburnt zones exhibited water repellency throughout the sampled profile (0–40 cm); however, the burnt zone affected by fire of medium–high severity exhibited decreased water repellency in the surface layer but increased at depths of 2–40 cm than it was both in the unburnt zone and in the zone burnt by low-severity fire.

► Soil water repellency was found not be an exclusively post-wildfire phenomenon.
► P. pinaster forest soils are extremely water repellent under dry conditions.
► Fire severity influences the thickness of the repellent layer in the burnt zones.
► Low-severity burning zone exhibited no significant differences from the unburnt zone.
► Medium-high severity burning zone was more repellent in depth that the unburnt zone.