## 3 Results and Analysis
### Spatial Variation Characteristics of Soil Salinity
#### 3.3.3 Nested Structure Analysis and Spatial Distribution Characteristics  
According to the previous analysis, the spatial variability of soil salt content under anisotropic conditions has strong directionality, and the influence of directionality on the fitting of variation function needs to be considered [21]. Since agricultural production activities mostly act on shallow soil, in this study, a single spherical model is used to  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Depth/m**} & \multicolumn{2}{c}{**Major Axis**} & **Major Axis Range** & **Minor Axis Range** & **Anisotropy** & **Nugget** & **Sill** \\  & **Direction Angle\({}^{\circ}\)** & _a/km_ & _a/km_ & **Ratio/k** & **C\({}_{0}\)** & **C\({}_{0}\) + C** \\ \hline \multirow{2}{*}{0–0.25 m} & 133 & 75.39 & 44.66 & 1.69 & 1.416 & 2.002 \\  & 43 & 44.66 & 75.39 & 0.59 & 1.416 & 2.002 \\  & 120 & 44.36 & 15.58 & 2.84 & 0.02 & 1.227 \\  & 30 & 15.58 & 44.36 & 0.35 & 0.02 & 1.227 \\  & 124 & 76.56 & 25.08 & 3.05 & 0.12 perform anisotropic nesting on the spatial structure of surface soil salt content. The collated nested model is shown below:  
\[\gamma(h)=\left\{ \begin{array}{cc}C_{0}&h=0\\ C_{0}+C\left(\frac{3h}{24}-\frac{h^{3}}{2\sigma^{3}}\right)&a>h>0\\ C_{0}+C&h>a\end{array} \right. \tag{8}\]  
\[\gamma^{*}(h)=\left\{ \begin{array}{cc}1.416&h=0\\ 1.416+0.586\left[\frac{8.07}{2}\left(\frac{h}{a}\right)-\frac{5.826}{2}\left( \frac{h}{a}\right)^{3}\right]&75.39>h>0\\ 2.002&h>75.39\end{array} \right. \tag{9}\]  
After fitting the variogram with the nested model, the ordinary Kriging interpolation method is used for spatial estimation of the surface sample data. The interpolation results are shown in Figure 7a It can be seen from Table 5 that the \(I\) and \(\left[\overline{Z(x_{i})}-Z(x_{i})\right]^{2}\) values of the nested model method are smaller than those of the ordinary Kriging interpolation method, which indicates that the spatial variation structure of soil salinity can be better expressed based on the nested model under anisotropic conditions and the interpolation accuracy can be improved. The soil salinity of layers between 1 m and 2 m and between 2 m and 3 m is plotted according to the ordinary Kriging interpolation method, and the results are shown in Figure 7b,c. According to the previous analysis, the surface soil salinity shows strong variability and directionality, indicating that its spatial distribution shape is irregular, as shown in Figure 7. The variation range of soil salinity in the layers between 0 m and 0.25 m is 0.213-1.775%, of which mild and partial moderately salinized are distributed in the north of the study area, i.e., inside the oasis (cultivation area), and extend to the southeast in an irregular ellipse. The oasis edge (i.e., oasis desert ecotone) is dominated by moderately salinized, which is distributed circularly. Part of the heavy salinization is distributed in the northeast of the study area, which is a low-lying basin with sparse vegetation and easy to store salt, the rest is distributed in belt along the Tarim River and at the edge of the oasis. On the whole, salinization presents a trend of gradual aggravation from the center to the edge of the oasis. When considering the continuity of spatial variation, the variation increase in salt content in the northwest southeast is relatively stable and the continuity is  
Figure 7: Spatial distribution of soil salt content at different depths: spatial distribution of soil salinity in the layers between 0 m and 0.25 m (**a**); spatial distribution of soil salinity in the layers between 1 m and 2 m (**b**); spatial distribution of soil salinity in the layers between 2 m and 3 m (**c**). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Interpolation Method** & \(I\)**Value** & \(\overset{\mathsf{A}}{[\boldsymbol{Z}(x_{i})-\boldsymbol{Z}(x_{i})]^{2}}\) \\ \hline Ordinary Kriging & 2.625 & 2.725 \\ Nested model method & 1.588 & 1.636 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of interpolation accuracy in different spaces.