## 2 Materials and Methods  
### Study Area Description  
Northern Vietnam is located at the intersection of two tectonic zones, the Tethys oceanic trench and Pacific Rim, and has undergone considerable uplifting and erosion [25]. The chronostrattigraphic stratum here is mainly comprised of Paleozoic marine sediment that formed shale and limestone. The Indochina orogenic cycle occurred 150 million years ago, during when the geosyncline uplifted gradually. This process pushed the carbonate layer upward, which formed the karst mountains 1000-2800 m tall. The SC profile is located at Sang Cai, Hoa Binh province, Northern Vietnam (20\({}^{\circ}\)46'11.35\" N, 104\({}^{\circ}\)52'29.26\" E) (Figure 1), at an altitude of 1135 m. The 400-700 m thick bedrock is gray and grayish-white dolomite under the Upper Dong Giao Formation (T\({}_{2}\)dg\({}_{1}\)). The overlay on the bedrock is yellowish-brown soil (Table 1), in which the rock-soil interface (RSI) is a grey-white powder layer 1-3 mm thick. T\({}_{2}\)dg\({}_{1}\) formation is the main karst stratum in Northern Vietnam. It is exposed at 30\({}^{\circ}\)-90\({}^{\circ}\) and has well-developed fractures. The formations are 5-40 km wide and 400 km long, stretching from the Sino-Vietnamese border northwest of the coastal area in the east [26]. Generally, the climate is tropical monsoon, with an annual average temperature of 24 \({}^{\circ}\)C, which is lower than at the same latitude in Asia. The average annual rainfall is 1500-2000 mm. The summer season is usually rainy and hot, affected by the southeast monsoon. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Layer** & **SC Profile** & **H (cm)** & **No.** & **TG Profile** & **H (cm)** & **No.** \\ \hline \multirow{4}{*}{A} & Blackish-brown topsoil layer & \(0\) & SC08 & Dark–gray topsoil layer & \(0\) & TG08 \\ \cline{2-6}  & yellowish–brown soil layer & \(-20\) & SC07 & Yellow soil layer & \(-30\) & TG07 \\ \cline{2-6}  & Yellowh–brown soil layer & \(-40\) & SC06 & Yellowh–brown soil layer & \(-90\) & TG06 \\ \hline \multirow{4}{*}{B} & Yellowh–brown soil layer & \(-60\) & SC05 & Yellowh–brown soil layer & \(-120\) & TG05 \\ \cline{2-6}  & Yellowh–brown soil layer & \(-80\) & SC04 & Yellowh–brown soil layer & \(-150\) & TG04 \\ \cline{2-6}  & Yellowh–brown soil layer & \(-100\) & SC03 & Yellowh–brown soil layer & \(-180\) & TG03 \\ \cline{2-6}  & Grey-white powder layer & \(-120\) & SC02 & Blackish–brown layer & \(-210\) & TG02 \\ \hline C & Dolomite & \(-130\) & SC01 & Limestone & \(-240\) & TG01 \\ \hline \hline \end{tabular}  
* A, B, and C are the soil genetic layers, corresponding to the topsoil, eluvial horizon, and bedrock, respectively; H, the depth of profile and its sampling points; SC and TG profiles are the dolomite and limestone weathered profiles in Northern Vietnam.  
\end{table} 
Table 1: Detailed information and sampling sites of the two typical laterite profiles*. Figure 1: The simplified stratigraphic sketch of the SC dolomite and TG limestone (located at Sang Cai, Hoa Binh province and located at Tong Gia, Lao Cai province, respectively, showing the location of the numbered sampling points. 1, plow layer; 2, terra rossa; 3, rock–soil interface (RSI) of dolomite rock (blackish-brown chocolate layer); 4, RSI of limestone (grey-white powder layer); 5, dolomite; 6, limestone; 7, sampling point. The TG profile is located at Tong Gia, Lao Cai province (22\({}^{\circ}\)29\({}^{\prime}\)1.01\" N, 104\({}^{\circ}\)06\({}^{\prime}\)57.10\" E) (Figure 1); at an altitude of 128 m. The bedrock of Ha Giang Formation (\(\rm\epsilon_{2}\)hg\({}_{2}\)) is comprised of marble, mottled limestone, clayish limestone, and clay-sericite schist, with a thick up to 1000 m. Red river valley stretches northwest\(-\)southeast and is known for its hot and dry features. A blackish-brown layer is found at the RSI of the TG profile. The sampling profiles were selected from the top of a hillock sited and manually excavated next to a highway. The weathering soil layer was about 100-300 cm.