Land‐Use‐Dependent Change in the Soil Mechanical Strength and Resilience of a Shallow Volcanic Ash Soil in Southern Chile

Core Ideas Land use change had a great impact on the spatial variability of Aquands' physical properties. Mechanical strength increased as a consequence of the land use change. The soil's physical resilience presents spatial variability. Aquands with a bulk density close to 0.65 g cm –3 can be considered to be compacted. Aquands are shallow volcanic ash soils, locally called Ñadi, which cover an area of 475,000 ha in Chile and have undergone intensive land use changes from secondary native forest (SNF) to naturalized grasslands (NG). This study analyzed the impact of this land use change in a Ñadi soil on the spatial changes in its mechanical strength and the consequences on pore functions and soil resilience. The change from SNF to NG occurred in 1980 because of a forest fire. We measured the volumetric water content and penetration resistance between 0 and 10 cm depth at 98 grid points across the domain (SNF and NG). Near each grid point, undisturbed soil core samples were collected to measure soil precompression stress (Pc), soil resilience after the application of mechanical stresses and water saturation via capillary rise, and the changes in air permeability (k a ). After the application of mechanical stresses (to 400 kPa), the soil presented a land‐use‐dependent soil deformation and a limited, spatially variable soil resilience, which homogenized the k a across the field. Despite a bulk density of 0.65 g cm –3 , these soils must be described as highly compacted and defined by high soil mechanical strength (Pc > 60 kPa) with small values of air capacity (<8%) and permeability (<1 log μm 2 ). To design proper schemes for land use planning of these specific soils, further analyses are required, as is a definition of a minimal Ñadi soil depth to support a productive system without detrimental effects on the ecosystem services.