## 2 Materials and Methods
### Soil Aggregate Fractionation  
In this study, four particle sizes were determined according to the classification standard of the International Soil Association [23; 24], namely, clay (<2 \(\upmu\)m), silt (2-20 \(\upmu\)m), fine sand (20-200 \(\upmu\)m), and coarse sand (200-2000 \(\upmu\)m). After the mixture of soil and water was fully dispersed ultrasonically, the soil samples were separated via a combination of wet sieving and sedimentation [23; 25; 26; 27]. Specifically, about 600 g of the air-dried sample was mixed and soaked in 2 L of deionized water for 24 h, before being subjected to ultrasonic vibration for 30 min. Coarse sand was separated from the suspension via wet sieving. The sieved suspension containing the fine fractions was placed in a sedimentation bucket before being diluted to 21 L. The sedimentation velocity of the clay and silt in still water was calculated according to [PERSON] law (Equation (1)), and then the sedimentation times, t\({}_{1}\) and t\({}_{2}\), were calculated according to the sedimentation distance, \(h\). Timing was initiated after the suspension was fully stirred. When the sedimentation time reached t\({}_{1}\), the siphon method was used to extract the suspension at a depth, \(h\), below the liquid level. Extraction was completed when the liquid level dropped to \(h\), then deionized water was added to achieve the original liquid level. This process was repeated several times until the upper  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **Slope/\({}^{\circ}\)** & \begin{tabular}{c} **Bulk Density/** \\ **(g\(\cdot\)cm\({}^{-3}\))** \\ \end{tabular} & \begin{tabular}{c} **pH/** \\ **(l\(\cdot\)2.5)** \\ \end{tabular} & \begin{tabular}{c} **OM/** \\ **(\%)** \\ \end{tabular} & \begin{tabular}{c} **TN/** \\ **(g\(\cdot\)Kg\({}^{-1}\))** \\ \end{tabular} & \begin{tabular}{c} **TP/** \\ **(g\(\cdot\)Kg\({}^{-1}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Olsen-P/** \\ **(mg\(\cdot\)Kg\({}^{-1}\))** \\ \end{tabular} &
\begin{tabular}{c} **TK/** \\ **(g\(\cdot\)Kg\({}^{-1}\))** \\ \end{tabular} \\ \hline A & 10 & 1.36 & 7.64 & 1.51 & 1.41 & 0.88 & 48.75 & 15.21 \\ B & 15 & 1.39 & 7.52 & 1.34 & 1.44 & 0.87 & 50.22 & 15.32 \\ \hline \hline \end{tabular}  
* pH: hydrogen ion concentration, OM: organic matter, TN: total nitrogen, TP: total phosphorus, Olsen-P: readily available phosphorus, TK: total potassium.  
\end{table}
Table 1: The basic physical and chemical properties of the soil sample.  
Figure 1: Location of sampling point in the Three Gorges Reservoir area.  
layer was completely clarified. The siphoned suspension was precipitated and dried to reveal clay. When this procedure was applied at t2, the soil sample siphoned out was silt. The remaining soil sample in the sedimentation bucket was fine sand. Each component was weighed after drying at 45 \({}^{\circ}\)C to constant weight, and the proportion of each particle fraction was calculated. Three parallel tests were carried out at the same conditions, after which soil samples of the same particle size were mixed.  
The relationship between the velocity and particle diameter is as follows [23]:  
\[v=\frac{(\rho_{s}-\rho_{l})gX^{2}}{18\eta} \tag{1}\]  
where \(v\) is the velocity of fall, \(X\) is the particle diameter, \(\rho_{s}\) is the particle density, \(\rho_{l}\) is the liquid density (water in this study), \(g\) is the acceleration due to gravity, and \(\eta\) is the fluid viscosity.