## 2 Materials and Methods
### Basic Approach  
To estimate the effect of road on soil, the transect line method was used for the soil sampling providing. Previous studies showed that the concentrations of heavy metals in roadside soils appear highest within 50 m from the road edge and reach the background values at 150 m [19; 25; 39]. Transect lines were taken as vertically as possible to the road, and the sampling distance was limited within 300 m off the road edge in the farmland and 200 m in the natural land area. Sampling intervals are small in areas within 50 m of the road and beyond 50 m, sampling distance is roughly 50 m apart. The soil type in this region is mainly red soil. The sample sites were chosen away from human constructions, such as gas station, service station, etc. The land use types included paddy field, grassland, dry land, natural forest, secondary forest, artificial forest and shrub land, and the total numbers of samples were 119, 10, 16, 30, 16, 18, 12, and 17 for each land use type, respectively.  
The natural, secondary and artificial forests are mainly tropical rain, mixed broad-leaved and rubber plantation forests, respectively. Due to the landscape heterogeneity in the study region, the sample sizes for each land use type were different. At each site, a \(10\times 10\) m quadrate was set to investigate the vegetation community characteristics. For the soil sites, the relative elevation and distance from the road were recorded. Soil samples were obtained using cores (5 cm diameter) from three soil profiles at each site. The soil was sampled at a depth of approximately 0-20 cm, and the three replicate samples were homogenized. Then, the soil samples were air-dried and sieved for the determination of soil nutrients. The soil characteristics examined for the surface layer included pH, soil organic matter (SOM), total nitrogen (TN), available potassium (AK), available phosphorus (AP), total potassium (TK) and total phosphorus (TP) by colorimetry after wet digestion with H\({}_{2}\)SO\({}_{4}\) and HClO\({}_{4}\)[40]. The determination of TN was by the semi-micro Kjeldahl method. SOM was determined by the K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) titration method after digestion [41]. AP and AK were extracted with a 3% (NH\({}_{4}\))\({}_{2}\)CO\({}_{3}\) solution. After filtering, the filtrate was measured by inductively coupled plasma atomic absorption spectrometry (ICP/AES). TK was determined by atomic absorption spectrometry. The metals investigated were Pb, As, Cr, Zn, Cd, Cu, and Ni. Although China has been using unleaded fuel for years, a major reservoir of lead in roadside soils still remains because the Pb present on the roadside from historical environmental emissions is stable and mainly concentrated in the surface layers. Soil samples were digested in a sand bath using a mixture of 3:1:1 nitric acid:perchloric acid:hydrofluoric acid at 160 \({}^{\circ}\)C over 6 h until no more fumes were liberated and the color of the mixture turned pale-yellow to colorless. After that, samples were filtered through filter papers, and the volumes were adjusted to 25 mL using distilled water. Finally, concentrations of these metals were measured by inductively coupled plasma atomic absorption spectrometry (ICP/AES).