A European Soil Organic Carbon Monitoring System Leveraging Sentinel 2 Imagery and the Lucas Soil Data Base

Kurzfassung

Worldsoils has developed a pre-operational Soil Organic Carbon (SOC) monitoring system in a cloud environment capable of: (i) predicting topsoil organic carbon content at regional and continental scales from Earth Observation (EO) satellite data with a continuous cover over Europe, (ii) leveraging upon multitemporal soil-spectral data archives and modelling techniques and, (iii) joining end users and EO experts for developing soil indices, relevant for monitoring topsoil at regional and continental scales. The system utilized spectral models for croplands and a digital soil mapping approach for permanently vegetated areas such as grasslands and forests. Models strongly rely on soil reflectance composites from the Sentinel 2 multispectral instrument. The composites provide the median reflectance for all valid pixels over a period of three years (from March to October). The bare soil frequency, a proxy for the degree of crop cover during the growing season, is clearly lower in a Mediterranean pilot region with winter cereals and fodder crops providing an extensive cover during the growing season. The graphical user interface provides SOC content and the 90% uncertainty ratio for 50 m pixels in three pilot regions and 100 m pixels for the rest of Europe. The system has been tested for three years and SOC prediction remain stable as to be expected for such a short period. The SOC prediction algorithms are reasonable compared to others at the continental scale (R²: 0.41 for croplands and 0.28 for permanently vegetated areas). Apart from tree crops in Macedonia (Greece) the soil reflectance composite attributes the correct model to validation sets of cropland and grassland in the pilot regions. The evaluation of the SOC prediction is satisfactory in Wallonia (Belgium; R² 0.51), but is hindered by the limited range of this attributes in croplands in Greece and the Czech Republic. Moreover, the bare soil frequency is lower in Greece as a result of abundant tree crops, cereals and fodder crops. The monitoring system is able to reproduce spatial patterns in SOC content similar to the ones obtained from a detailed regional algorithm using the new generation of hyperspectral satellites. Although the very high values in kettle holes in a morainic landscape of Northern Germany are underestimated.