Effect of plant ash incorporation on hydrologic processes of coarse-textured soils

Wildfires significantly affect soil hydrologic processes, particularly in coarse-textured soils common in fire-prone areas such as central Chile. This study evaluated the hydrological effects of incorporating plant-derived ash into coarse-textured soils using laboratory measurements and HYDRUS-1D modeling. Ashes from Eucalyptus globulus (exotic) and Quillaja saponaria (native) were incorporated at 4% by weight into sandy loam (SL) and sandy clay loam (SCL) soils, simulating mixing into the upper 5 cm of a 30 cm profile. Compared to soils without ash, Q. saponaria increased saturated water contents by 12% in SL and 9% in SCL, while saturated hydraulic conductivity (Ks) decreased by 82% in SL with E. globulus and by 69% in SCL with Q. saponaria. Ash effects were concentrated near the surface, at 2.5 cm depth, volumetric water content (θ) increased up to 42% in SL with Q. saponaria and 21% in SCL with E. globulus during the rainy season. Simulations showed that, for similar depths, ash-amended soils retained higher θ despite lower pressure heads (more negative h), indicating enhanced moisture availability. Rosetta-based scenarios overestimated θ, as confirmed by RMSE analysis. These results underscore the value of process-based modeling for assessing post-fire soil behavior and the hydrological role of ash incorporation.