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Additional file 1. Fig. S1. Overview of the modified BCR sequential extraction method for REE in this study. Fig. S2. The relationships of the values of LaN/GdN of carbonate-bound and reducible with mean annual precipitation (R2carbonate-bound = 0.2, P < 0.05; R2reducible = 0.9, P < 0.01).

Additional file 2. Table S1. The parameters and bulk REE concentrations (��g g-1 soil) in topsoils (0���10 cm) along the transect. Locations of the sites (S01���S21) are plotted in Fig. 1. Table S2. The mineral composition (%) in topsoils (0���10 cm) along the transect. Locations of the sites (S01���S21) are plotted in Fig. 1. Table S3. The fractional and bulk REE concentrations (��g g-1) in topsoils (0���10 cm) along the transect. Locations of the sites (S01���S21) are plotted in Fig. 1. Table S4. The REE geochemistry in different soil components (carbonate-bound, reducible, oxidizable, residue) in topsoils (0���10 cm) along the transect. Locations of the sites (S01���S21) are plotted in Fig. 1. Table S5. The parameters and REE concentrations (��g g-1 soil) along the soil profile (0���10, 10���20, 20���40, 40���60, 60���100 cm) at selected sites (S03, aridity index (AI)=0.03; S09, AI=0.10; S13, AI=0.22; S19, AI=0.51). Table S6. The fractional and bulk REE concentrations (��g g-1 soil) along the soil profile (0���10, 10���20, 20���40, 40���60, 60���100 cm) at selected sites (S03, aridity index (AI)=0.03; S09, AI=0.10; S13, AI=0.22; S19, AI=0.51). Table S7. The parameters of geochemical REE (carbonate-bound, reducible, oxidizable, residue) along the soil profile (0���10, 10���20, 20���40, 40���60, 60-100 cm) at selected sites (S03, aridity index (AI)=0.03; S09, AI=0.10; S13, AI=0.22; S19, AI=0.51).