Natural forest conversion to exotic pine plantations induces soil mineralogical and physicochemical changes – implications for soil organic carbon stabilization

Extensive areas of native forests worldwide have been converted into exotic tree plantations, generating environmental impacts, including loss of biodiversity and alteration of critical hydrological and biogeochemical functions. Yet, the effect of this land-use change (LUC) on soil properties, especially mineralogy and carbon stabilization potential, has received less attention. This research assesses the impacts of converting former native temperate deciduous forests (Nothofagus) to evergreen conifer (pine) plantations on soils using paired plots. We compared soils to a depth of 2.4 m at five sites in south-central Chile under similar climates but contrasting soil types and parent materials, ranging from residual soils of crystalline rocks (schist [SCH-Ultisol] and granite [GR-Alfisol]) to pyroclastic deposits of different ages (recent ash [RA-Entisol], young ash [YA-Andisol], and old ash [OA-Alfisol]). Compared to native forests, plantation soils were more acidic, stored less organic carbon (SOC), and had lower exchangeable cations and higher exchangeable Al 3+ . Developed soils from pyroclastic deposits (OA-Alfisol and YA-Andisol) showed small changes in pH and Fe-Al near-crystalline phases. Conversely, well-developed soils from crystalline parent materials (GR-Alfisol and SCH-Ultisol) and weakly developed volcanic soils (RA-Entisol) were less resilient to this LUC, displaying changes in most physical and chemical properties, including changes in the relative abundances of near-crystalline and amorphous phases. Acidification, SOC losses, soil desiccation, and Al translocation were identified among the main processes modified by pine plantations. We found that soils with more substantial shifts in crystalline minerals (GR-Alfisol and YA-Andisol) were also more depleted in SOC and Fe-Al organometallic and amorphous phases at deeper horizons. Our findings highlight the delicate, mineral-dependent processes that stabilize and protect organic matter over decadal timescales. The substantial impact of forest conversion on mineralogical and physicochemical properties at greater soil depths than previously reported underscores the need to consider deeper soil compartments when evaluating LUC effects. • Forest conversion to exotic pines alters soil mineralogy and physicochemical properties. • Plantation soils have lower pH, SOC, exchangeable cations, and higher exchangeable Al 3+ . • Acidification, SOC losses, Al release and translocation, and soil dehydration are identified as key processes. • Soils with substantial changes in crystalline phases exhibit greater C losses. • Mineralogical alterations extend well into deeper soil horizons (1.0 to 2.4 m depth).