After the samples were mixed thoroughly, we collected 1 kg of each sample; 1297 topsoil (0-20 cm) samples were collected in this study ( Figure 1). The GPS **data** of all samples were recorded before samples were air-dried naturally at room temperature. After removing plant residues and stones, all samples were passed through a 100-mesh nylon sieve (0.2 mm). The determination of soil total nitrogen (TN) was measured by the method described by Walkley and Black [30]; the soil available phosphorus (AP) was extracted by 0.5 mol L −1 NaHCO 3 and then measured via the Mo-Sb colorimetric method; and soil available potassium (AK) was extracted by 1 mol L −1 NH 4 -OAc with 1:5 weight-to-volume ratio and then measured by flame photometry [31].