Effect of fire severity on water repellency and aggregate stability on Mexican volcanic soils

A field study was conducted in order to study the effects of different wildfire severities on [1] soil organic matter content, [2] soil water repellency, and [3] aggregate stability; [4] the distribution of soil water repellency in aggregate sieve fractions (1–2, 0.5–1, 0.25–0.5 and < 0.25 mm) was also studied. Five similar burned sites and two long-unburned control sites were selected under mixed fir and pine forests in volcanic highlands from Michoacán, Mexico. Soil water repellency was observed in soil samples from all sites, although changes were influenced by fire severity. Sites affected by low severity fires did not show important changes in burned soils in comparison with controls, while high severity fires caused different responses: water repellency was increased or destroyed probably due to temperatures below or above 200–250 °C during burning. The degree of wettability/repellency from the fine earth fraction of burned soils seems to be conditioned by < 0.5 mm aggregates, more than coarser aggregates which always showed a higher degree of wettability. It is suggested that destruction of organic matter during burning occurs principally in coarse aggregates, where combustion can be more intense. Aggregate stability (measured using pre-wetted aggregates between 4 and 4.8 mm) did not change under low severity burning but it was considerably reduced in the case of a high fire severity. Losses of organic matter and destruction of water repellency seem to be the reasons for that reduction in this type of soil in contrast to previous studies, where aggregate stability increased after burning. Changes in both properties (water repellency and aggregate stability) are expected to induce modifications in runoff and soil loss rates at the hillslope scale.

Research Highlights
► Volcanic soils under pines and firs in the study area show a natural water repellency background.
► Different fire severities cause different responses.
► Post-fire water repellency is due to concentration of hydrophobic substances in finer aggregates.
► Fire-induced patchy pattern of water repellency originates complex hydrological responses.