Immediate effects of wildfires on water repellency and aggregate stability in Mediterranean calcareous soils

Alkaline soils are considered much less prone to developing water repellency induced by fire than acidic soils. Here we report on the persistence of water repellency present in calcareous soils immediately after wildfires in 10 burned areas in SE Spain, its distribution in different aggregate size fractions (<2, 2-1, 1-0.5, 0.5-0.25 and <0.25 mm) and on results from aggregate stability tests. We also distinguished between soil samples taken beneath pine (Pinus halepensis) and beneath understory vegetation. Burning appears to have increased the frequency of water repellency occurrence, with 74% of burned samples being classified as water repellent compared to 33% from unburned terrain in the composite fraction (<2 mm). The persistence of water repellency after fire was highly variable but nevertheless showed statistical differences at p<0.001 comparing burned vs unburned. Moreover, statistical differences in the persistence of water repellency were found in soil samples taken from beneath pine between burned and unburned, and also beneath understory vegetation. Results showed that soil beneath understory tend to have lower values of water repellency persistence than soil beneath pine. Burned soils showed higher aggregate stability. A positive tendency was observed between aggregate stability and water repellency. The observed changes in WR and AS could have implications for soil hydrological behaviour. (C) 2008 Elsevier B.V. All rights reserved.