Topsoil microstructure changes after a shrubland prescribed burn (Central Pyrenees, NE Spain)

The dense thicket Echinospartum horridum (Vahl, Rothm) is expanded in secondary pastures of the Central Pyrenees (NE-Spain). The control of this grassland encroachment is attempted through prescribed burnings, trying to minimize its direct effects on the soil. But the structural changes on the new soil surface, burned and bare, are unknown in the medium-term. To check it, soil aggregate stability (SAS), mean weight diameter of the aggregates (MWD), water repellency (WR), unsaturated hydraulic conductivity (k), and soil organic carbon (SOC) were measured in the surface (at 0-1, 1-2, 2-3, and 3-5 cm) in both unburned and 1-yr burned soils, after verifying that it suffered no direct damage. We also used the digital images of thin sections, obtained from undisturbed and oriented topsoil samples, to detect potential changes in soil microstructure. No significant changes were found in SAS, MWD and SOC for any thickness of soil studied. Nevertheless the WR, which was high before and just after burning, decreased significantly in the upper soil cm after 1-yr burning. WR decrease coincides with the 6-fold increase of the unsaturated hydraulic conductivity (k) and the presence of cappings on the burned topsoil. Cappings are coatings poor in organic matter and composed by fine sand-sized particles of angular quartz, mixed with charcoal, covering irregularly the original topsoil. The formation of cappings seems to derive from the impact of raindrops on the bare soil surface, hence its irregular spatial distribution. Summarizing, removing bushes by means of a low-intensity fast-moving prescribed burning caused the formation of discontinuous cappings without worsening significantly the rest of the measured properties.

Keywords: Aggregation; Cappings; Fire; Infiltration; Micromorphology; Water repellency.