Fig. 1. Distribution of the study area and sampling points

Fig. 2. PCA-Mahalanobis distance distribution

Fig. 3. Original spectra and the pretreated soil spectral reflectance curves. (a) Original spectral reflectance; (b) spectral reflectance after SG smoothing; (c) spectral reflectance corrected for multiple scattering; (d) spectral reflectance treated with first order differentiation

Fig. 4. Contribution diagram of the first 10 variables. (a) Original spectral reflectance; (b) spectral reflectance after SG-MSC treatments; (c) spectral reflectance after SG-MSC-FD treatments

Fig. 5. Comparison of soil organic matter prediction variables

Fig. 6. Correlation between different soil parameters (n=101), in which the curves are fitting curves

Fig. 7. Correlation between SOM, EC, Fe and pH and original spectral reflectance (n=101)

Fig. 8. Correlation between soil organic matter and the first five principal components for original spectrum and preprocessed spectra under two spectral treatments of SG-MSC and SG-MSC-FD

Fig. 9. Fitting scatter diagrams of PLSR model under three strategies. (a) Model 1; (b) model 2; (c) model 3; (d) model 4; (e) model 5; (f) model 6; (g) model 7

Fig. 10. VIP values of prediction variables in different PLSR models. (a) Model 3; (b) model 4; (c) model 5