## 3 Results and Discussion
### Soil Quality Assessment  
#### 3.2.1 Comparison to National Agricultural Land Risk Control Standard Values  
The number of sites and the percentage from which the collected surface soils registered values of PTEs (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) that exceeded the risk screening \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Parameters** & **As** & **Cd** & **Cr** & **Cu** & **Hg** & **Ni** & **Pb** & **Zn** & **pH** \\  & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **pH** \\ \hline Depths (cm) & 0.-20 & 0.-20 & 0.-20 & 0.-20 & 0.-20 & 0.-20 & 0.-20 & 0.-20 & 0.-20 \\ N & 1574 & 1574 & 1574 & 1574 & 1574 & 1574 & 1574 & 1574 & 1574 \\ Mean & 23 & 0.27 & 128 & 49 & 0.18 & 58 & 45 & 115 & 6.45 \\
25 th & 10 & 0.14 & 91 & 32 & 0.07 & 40 & 27 & 84 & 5.5 \\ Median & 16 & 0.19 & 113 & 41 & 0.1 & 51 & 34 & 100 & 6.46 \\
75 th & 27 & 0.28 & 151 & 58 & 0.17 & 70 & 43 & 121 & 7.38 \\ Min & 0.5 & 0.02 & 10 & 3.12 & 0 & 3.68 & 4.42 & 28 & 4.13 \\ Max & 909 & 5.35 & 422 & 652 & 6.81 & 361 & 2106 & 1943 & 8.33 \\ Standard deviation & 31.21 & 364.21 & 53.3 & 32.43 & 377.99 & 28 & 93.48 & 93.76 & 1.05 \\ Variance & 973.78 & 132.646.72 & 2841.27 & 1051.42 & 1425.577.16 & 772.12 & 8737.68 & 8791.49 & 1.1 \\ Skewness & 15.83 & 7.92 & 1.32 & 7.51 & 10.68 & 2.17 & 16.07 & 11.74 & \(-0.07\) \\ Kurtosis & 417.65 & 83.5 & 2.18 & 108.21 & 145.11 & 12.34 & 298.38 & 185.29 & \(-1.2\) \\ Local background \({}^{\ast}\) & 19 & 0.13 & 119 & 45 & 0.11 & 58 & 31 & 100 & 6.57 \\ Regional background \({}^{\ast}\) & 15.6 & 0.083 & 67.9 & 35.1 & 0.05 & 40.9 & 37.1 & 88.4 & - \\ National background \({}^{\ast}\) & 9 & 0.11 & 50 & 19 & 0.018 & 22 & 21 & 60 & 8.1 \\ Australian surface soil \({}^{\ast}\) & 1.6 & 0.04 & 23.6 & 11.2 & 0.01 & 9.8 & 7.22 & 26.1 & - \\ American soil A layer \({}^{\ast}\) & 5.2 & 0.02 & 31 & 14.8 & 0.02 & 13.8 & 17.8 & 59 & - \\ Surface soil of Europe \({}^{\ast}\) & 7.03 & 0.145 & 60 & 13 & 0.037 & 18 & 22.6 & 52 & 5.51 \\ Surface soil of Sacramento Valley northern & 6 & 0.2 & 96 & 37 & - & 50 & 23 & 88 & - \\ California \({}^{\ast}\) & - & - & 0.237 & 31 & - & 1549 & 23 & 63 & - \\ Surface soil from 72 examined mines of China \({}^{\ast}\) & 195.5 & 11 & 84.28 & 211.9 & 3.82 & 106.6 & 641.3 & 1163 & - \\ Surface soil near Pb-Zn mine \({}^{\ast}\) & 316.67 & 0.597 & 77.57 & 0.065 & 680 & 37.69 & 362 & 486 & - \\ Surface soil Guangqi Hengüan \({}^{\ast}\) & 75.8 & 1.91 & 467.0 & 48.5 & 0.21 & 76.2 & 84.2 & 258.2 & 6.1 \\  & 30 & 0.3 & 150 & 50 & 0.5 & 60 & 70 & 200 & pH \(\leq\) 5.5 \\  & 30 & 0.3 & 150 & 50 & 0.5 & 70 & 90 & 200 & 5.5 \(\pm\) pH \(\leq\) 5.5 \\ RSV of China \({}^{\ast}\) & 25 & 0.3 & 200 & 100 & 0.6 & 100 & 120 & 200 & 6.5 \(\pm\) pH \(\leq\) 7.5 \\  & 20 & 0.6 & 250 & 100 & 1 & 190 & 170 & 300 & pH \(>\) 7.5 \\  & 200 & 1.5 & 800 & - & 2 & - & 400 & - & pH \(\leq\) 5.5 \\ RIV of China \({}^{\ast}\) & 150 & 2 & 850 & - & 2.5 & - & 500 & - & 5.5 \(\pm\) pH \(\leq\) 6.5 \\  & 120 & 3 & 1000 & - & 4 & - & 700 & - & 6.5 \(\pm\) pH \(\leq\) 7.5 \\  & 100 & 4 & 130 & - & 6 & - & 1000 & - & pH \(>\) 7.5 \\ \hline \hline \end{tabular} Notes: “-” not available; \({}^{\ast}\): the median values of 425 deep soils in study area [54]; \({}^{\ast}\): the median values of deep soil of Yunnan province [88]; \({}^{\ast}\): the median value of deep soil of China geochemical baseline project [87]; \({}^{\ast}\): reference from [89]; \({}^{\ast}\): reference from [90]; \({}^{\ast}\): reference from [91]; \({}^{\ast}\): reference from [60]; \({}^{\ast}\): reference from [94]; \({}^{\ast}\): reference from [5]; \({}^{\ast}\): reference from [92]; \({}^{\ast}\): reference from [93]; \({}^{\ast}\): the risk screening values for soil contamination of agricultural land of China [95]; \({}^{\ast}\): the risk intervention values for soil contamination of agricultural land of China [95].  
\end{table}