Abstract
• Limited bare soil conditions restrict conventional EO-based SOC monitoring • Farm management strongly controls bare-soil frequency and SOC predictability • Deep-learning satellite embeddings provide top SOC prediction • Spatially informed models help capture SOC variability across regions • SOC assessment is feasible without relying solely on bare-soil imagery The estimation of Soil Organic Carbon (SOC) from optical image spectroscopy typically relies on the availability of bare-soil conditions, which are increasingly rare due to the widespread adoption of conservation agriculture practices. This study evaluates alternative strategies for SOC prediction under limited bare-soil availability by comparing four methodological approaches based on Sentinel-2 imagery and related products: (i) bare-soil multispectral composites, (ii) vegetation indices, (iii) AlphaEarth Satellite Embeddings, and (iv) a hybrid geostatistical–machine learning model (KpR–Cubist). These methods were tested across three cropland regions with contrasting pedoclimatic conditions: Italy, France, and Taiwan. The evaluation relied on more than 1,800 topsoil samples collected between 2020 and 2024. Results show that bare-soil availability varies significantly by region, with cloud cover and vegetation/farm management being the main limiting factors. Models using Satellite Embeddings consistently achieved the highest predictive accuracy (RPIQ up to 2.24) , outperforming conventional bare-soil composite and vegetation-based models. Incorporating spatial coordinates further improved model performance, revealing strong spatial autocorrelation in SOC distribution. The hybrid kriging–Cubist approach achieved comparable accuracy to the embedding-based models, confirming the value of integrating spatial dependence into data-driven frameworks. Overall, the study demonstrates that deep-learning–derived satellite embeddings models provide effective alternatives for SOC estimation in croplands where bare-soil imagery is increasingly unavailable due to sustainable soil management practices.
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