Assessing Soil Organic Carbon from Sentinel-2 Imagery and Regional Calibration Approach in Salt-Affected Agricultural Lands: A Case Study from Portugal

Mapping Soil Organic Carbon (SOC) at a regional scale is essential for assessing soil health and supporting sustainable land management. This study evaluates the potential of using Sentinel-2 imagery and regional calibration to predict SOC in salt-affected agricultural lands in Portugal, while also assessing the influence of soil properties, such as salinity, on SOC prediction. A per-pixel mosaicking approach was set to analyze the relation of spectral reflectance indices linked to bare soil conditions with SOC. SOC prediction models were developed using Linear Regression (LR) and Partial Least Squares Regression (PLSR). Among the tested approaches, the combination of maximum Bare Soil Index (maxBSI) with LR produced the most accurate SOC predictions, achieving moderate prediction performance (R² = 0.52, RMSE = 0.16%). This approach slightly outperformed the application of the 90th percentile of bare soil pixels (R90 reflectance) and the median approaches with PLSR. Notably, our findings indicate that soil salinity did not significantly affect SOC predictions, suggesting that within the observed salinity range of ECe between 1.2 and 10.4 dS m⁻¹ in topsoil, salinity had no statistical influence on SOC prediction. However, further case studies are needed to validate this observation across diverse agricultural conditions. In contrast, soil texture and moisture content emerged as the dominant factors influencing soil reflectance. This study demonstrates that Sentinel-2-derived indices, particularly maxBSI, combined with the proposed regional calibration approach, can be a cost-effective and scalable solution to monitor and regularly update SOC maps.