## 3 Results
### Root-Soil Strength Test  
#### 3.3.1 Shear Strength and Increment  
This study identified clear interspecific differences in the responses of plant root-soil composites to varying durations of flooding stress. The shear strength of the root-soil composite varied distinctly among species (Figure 7A, Table 5). _Carex_ exhibited an initial increase in shear strength during the early flooding period, followed by a gradual decrease after 120 days. Despite the reduction under prolonged flooding, the overall decline in shear strength was moderate. In contrast, _Cynodon_ showed the highest initial shear strength within 60 days of flooding; however, unlike _Carex_, its shear strength consistently declined throughout the entire flooding period. The decrease in shear strength was steady and gradual, resulting in a total reduction of 19.27% after 180 days. _Eleusine_ demonstrated the most dramatic decrease in shear strength, dropping significantly after 30 days of flooding and eventually approaching values similar to the CK group.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Species** & **Parameters** & **Equation** & **R\({}^{2}\)** & **Significance** \\ \hline \multirow{2}{*}{_Carex_} & Shear strength & \(y=49.43+7.2\times 10^{-2}x-4.38\times 10^{-4}x^{2}\) & 0.82 & \textless{}0.05 \\  & Strength increment & \(y=27.84+2.86\times 10^{-3}x-2.09\times 10^{-5}x^{2}\) & 0.97 & \textless{}0.01 \\  & Shear strength & \(y=52.80-5.01\times 10^{-3}x-1.87\times 10^{-4}x^{2}\) & 0.98 & \textless{}0.01 \\  & Strength increment & \(y=15.49+0.18x-8.63\times 10^{-4}x^{2}\) & 0.84 & \textless{}0.01 \\  & Shear strength & \(y=52.50-0.27x+6.79\times 10^{-4}x^{2}\) & 0.96 & \textless{}0.01 \\  & Strength increment & \(y=15.16-0.07x-3.78\times 10^{-5}x^{2}\) & 0.74 & \textless{}0.05 \\  & Shear strength & \(y=37.31-0.19\times 10^{-2}x+6.78\times 10^{-4}x^{2}\) & 0.96 & \textless{}0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Fitting equation for soil shear strength and flooding duration.  
Figure 6: Effect of diameter on root elastic modulus under different flooding treatments of plants: (**A**) represents _Carex_; (**B**) represents _Cynodon_; (**C**) represents _Elusine_; the range of 0 to 180 indicates the number of days for various flooding treatments.  
Figure 7: Effect of varying flooding durations on soil strength in plants: (**A**) represents shear strength; (**B**) represents strength increment; CK is the no-plant flooding control and the soil shear strength map with a vertical pressure of 50 kPa.  
In a flooded environment, the presence of vegetation can significantly enhance the soil's shear strength. During the initial stages of flooding, all three types of plants can effectively contribute to an increase in soil strength. Specifically, the effects of _Carex_ and Zoysia on the improvement of soil shear strength initially rise with the duration of flooding, followed by a decline; in contrast, Paspalum exhibits a gradually decreasing trend after the initial flooding period. After 120 days, 90 days, and 30 days of observation, _Carex_, Zoysia, and Paspalum achieved peak values of soil shear strength increase, measuring 27.59, 24.74, and 17.78 kPa, respectively (Figure 7B).