Influence of wildfire severity on soil physical degradation in two pine forest stands of NW Spain

• Low severity fires do not markedly influence soil erodibility.
• Wildfire-induced changes in aggregate stability are highly variable.
• Use of controlled heating experiments to infer the severity of wildfires
• Fire severity is a key explanatory variable and should be estimated in future field studies.

This study examined the effect of wildfire on the physical degradation of two forest soils in NW Spain and compared this against changes produced by controlled laboratory heating experiments in which unburned soils from neighbouring sites were exposed to five different heating regimes. The physical soil degradation was considered with respect to organic carbon content, dry aggregate size distribution, water aggregate stability, water repellency, and hydraulic conductivity. The results of this analysis show that the changes in the physical properties of the wildfire-affected soils were similar to the changes observed in the heating experiments. However, the wildfire had contrasting effects in terms of soil physical degradation at the two study sites, which can be attributed to differences in soil heating regimes and fire severity. At one study site, the wildfire did not produce marked changes in organic carbon content or in aggregation, except for a slight fragmentation of macro-aggregates into micro-aggregates. Also, soil water repellency remained very strong after the wildfire, resulting in low hydraulic conductivity. Based on the laboratory experiments, the soil at this site was not exposed to temperatures above 220 °C. The wildfire at the other study site resulted in more pronounced degradation of the soil, as shown in the reduction of organic carbon content by almost 50%, resulting in reduced stability of the aggregates. The wildfire also reduced soil water repellency considerably, but hydraulic conductivity did not increase noticeably. In this case the soil could be exposed to temperatures between 220 and 380 °C.