Widespread depletion of inorganic carbon in Europe's topsoil revealed by spatiotemporal digital soil mapping

As a critical yet overlooked carbon pool, soil inorganic carbon (SIC) links organic-inorganic processes in the global carbon cycle. However, large-scale assessments of SIC changes remain limited, restricting our knowledge of how and why SIC varies over years across broad spatial scale. In this study, we assessed the spatial distribution and temporal trends of surface SIC across the European Union and the United Kingdom using spatiotemporal digital soil mapping with 13,206 repeated Land Use and Coverage Area Frame Survey (LUCAS) samples (2009, 2015, and 2018). We developed a binary classification random forest (RF) model to detect SIC presence (area under curve = 0.94, F1 score = 0.90) followed by a regression RF model to predict SIC content (R² = 0.57, Lin's consistent correlation coefficient = 0.64). The results revealed that surface SIC showed an average decline of 8.71 % from 2009 to 2018, with significant losses in Southern and Central Europe, reaching up to 50 % in coastal areas, while Spain’s Meseta Plateau increased. Spatially, SIC was concentrated in arid and semi-arid southern Europe, especially calcium-rich soils, but was nearly absent in humid northern soils. Shrubland exhibited the largest reductions of SIC, followed by cropland, while grassland and bare land showed slight increases. Soil pH emerged as the main driver of SIC changes, followed by Normalized Difference Vegetation Index, Topographic Wetness Index, Palmer Drought Severity Index, slope, and soil type. The study emphasizes the necessity of tracking SIC dynamics and supports the development of soil health monitoring frameworks and climate-smart land use strategies.