Results
Physicochemical characteristics of sand samples
In the surface sand samples, the physicochemical parameters displayed significant differences (P < 0.05), except Zn and TC, (Table 1 and Table S2). The pH, altitude, Ca2+, P and Y showed highly significant differences (P < 0.001). In the subsurface sand samples, the physicochemical parameters exhibited significant differences (P< 0.05) except WC, EC, TN, TOC, Cl-, Mg2+ and Cl, (Table 1 and Table S2). The altitude, P, Ti, Mn, V and Y showed highly significant differences (P< 0.001). Overall, the soil pH and altitude significantly decreased from north to south (P < 0.05). Furthermore, the physicochemical parameters of surface sand were similar to that of the subsurface sand, except the WC and Y levels which exhibited significant differences between the surface and subsurface samples.
Variations of bacterial communities in the samples
16S rRNA gene sequencing generated a total of 2160932 sequences after quality control. The sequences were classified into 5,195 OTUs at the 97% similarity level. In the surface samples, the Shannon index, Chao index, Simpson index, Sobs index and Ace index of the samples ranged from 0.807 to 3.044, from 151 to 312, from 0.133 to 0.807, from 138 to 278, and from 151 to 319, respectively (Table 2). Shannon and Simpson indices showed significant differences in the surface samples. In the subsurface samples, the Shannon index, Chao index, Simpson index, Sobs index and Ace index of the samples ranged from 0.790 to 4.111, from 158 to 431, from 0.058 to 0.816, from 137 to 335, and from 149 to 427, respectively. All of these indices showed significant differences in the subsurface samples. No significant differences were observed between the surface alpha diversity and that of the subsurface.