Fig. 1. Overview of the study area and distribution of sampling points

Fig. 2. Soil primary spectra obtained by different methods. (a) Laboratory-measured primary spectra; (b) field-measured in situ primary spectra; (c) Sentinel-2 primary spectra

Fig. 3. Reflectance curves of indoor spectra for soil after different mathematical transformations. (a) FD; (b) SNV; (c) Log; (d) SG

Fig. 4. Correlation coefficient curves for SOC content with spectra transformed by different methods. (a) RSR; (b) FD; (c) Log; (d) SNV; (e) SG

Fig. 5. Accuracies comparison of different combination models. Modeling results of field-measured in situ spectra by (a) XGBoost, (b) RF, and (c) PLSR; modeling results of indoor spectra by (d) XGBoost, (e) RF, and (f) PLSR; modeling results of Sentinel-2 spectra by (g) XGBoost, (h) RF, and (i) PLSR

Fig. 6. Scatter plots of measured values and predicted values for combination models after characteristic band selected for spectra with FD transform. Modeling results of field-measured in situ spectra by (a) XGBoost, (b) RF, and (c) PLSR; modeling results of indoor spectra by (d) XGBoost, (e) RF, and (f) PLSR

Fig. 7. Land use/land cover types and the extracted bare soil information from Sentinel-2 imagery in the study area. (a) Image of land use/land cover types; (b) image of bare soil information

Fig. 8. Distribution of SOC content based on Sentinal-2 imagery in the study area

Fig. 9. Digital elevation map and soil type map of the study area. (a) Digital elevation map; (b) soil type map