Post-fire soil water repellency under stones and forest residue mulch versus of bare soil

Soil water repellency (SWR) is commonly defined as a physical property of soil to resist wetting. Fire can induce, enhance, or reduce SWR and, consequently, lead to considerable changes in soil water infiltration and storage and increase soil erosion by water. The application of mulches to cover burned areas has been found to be an efficient emergency stabilization treatment. However, little is known about possible side effects on SWR, especially long-term effects. Under forests, SWR is known to be very heterogeneous, particularly in proximity to trees and shrubs, litter type and thickness, stones, cracks and roots. This study targeted the effects of post-fire mulching on SWR in a eucalypt plantation five years after a wildfire. The application of forest residue mulch did not significantly change SWR in bare soil patches or under stones, comparing the mulched and untreated plots. By contrast, SWR in the mulched plots was, significantly stronger under mulch than in bare soil. The same was true for both soil organic matter content (SOM) and soil moisture content (SMC), suggesting that SOM played a more important role than SMC. In turn, SWR under mulch was not significantly different from SWR under stone, while both SMC and SOM were significantly higher under mulch than stone. This could be explained by the differences in SMC overriding the effects of the differences SOM, or, alternatively, by possible differences in SOM quality, in particular of the “fresh” input from the mulch. Overall, the present results indicated that different mechanisms may drive SWR dynamics beneath mulch fragments, stones and bare soil patches. A better understanding of these mechanisms is important to improve the knowledge of post-fire overland flow generation and, thereby, to improve its prediction using hydrological models, especially during the early phases of the window-of-disturbance.