## 3 Results and Discussion
### Genesis of Low As Bioavailability in Paddy Soils in Karst Areas  
The computed Pearson's correlation values among the soil properties, residual As (F7), and the total soil As are shown in Table 7. Both the total soil As and F7 showed positive correlations with TFe\({}_{2}\)O\({}_{3}\), Mn, and Al\({}_{2}\)O\({}_{3}\), and they showed negative correlations with SiO\({}_{2}\). Notably, previous research employing field-emission scanning electron microscopy reported that the basic elements in Fe-Mn nodules were Fe, Mn, Al, and Si [28,35]. Guangxi encompasses the largest and most distinctive karst landform worldwide [74]. Temperature and rainfall intensify the pedogenesis of carbonate rocks, which includes two stages: leaching accumulation and weathering [27]. The bulk of alkali metals and alkaline earth metals, such as Ca, Mg, and K, leaches from carbonate rocks, whereas Fe, Mn, and Al are retained in and form into (oxyhydr)oxides and minerals, which strongly adsorb As and trace metals, resulting in their comparative enrichment [6,34,71]. These hydrous oxides and some clay minerals gradually dissolve and are repeatedly deposited with \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fraction**} & \multicolumn{4}{c}{**Proportion (\%) a**} \\  & **Min** & **Max** & **Median** & **Mean** \\ \hline Water-soluble (F1) & 0.02 & 7.80 & 0.38 & 0.56 \\ Exchangeable (F2) & 0.01 & 3.34 & 0.37 & 0.51 \\ Bound to carbonates (F3) & 0.03 & 5.76 & 0.75 & 0.97 \\ Weakly bound to organic matter (F4) & 0.48 & 45.03 & 10.21 & 11.73 \\ Fe/Mn oxide-bound (F5) & 1.11 & 22.49 & 7.35 & 8.07 \\ Strongly bound to organic matter (F6) & 0.03 & 7.10 & 0.79 & 1.19 \\ Residual (F7) & 37.06 & 94.56 & 78.74 & 76.96 \\ \hline \hline \end{tabular}  
* Proportion (%): Proportion of each fraction to the sum of the seven fractions.  
\end{table} 
Table 6: The proportion of As in each fraction by the sequential extraction procedure (\(N\) = 305). Figure 4: Dendrograms of As in soils and rice grains. F1, F2: water-soluble and exchangeable; F3: bound to carbonates, F4: weakly bound to organic matter, F5: Fe/Mn oxide-bound, F6: strongly bound to organic matter, F7: residual. \end{table}
Table 7: Pearson’s correlation coefficients between the soil properties and residual As (F7) (\(N\) = 305). alternating oxidation and reduction, ultimately forming Fe-Mn nodules [75; 76]. During later pedogenesis, Fe-Mn nodules are progressively exposed to the ground, along with the strengthening of weathering [38]. The research findings indicate that Fe-Mn nodules accounted for 26.21% of the total amount of soil, and those with diameters lower than 2 mm accounted for 14.18% of the total soil mass in karst areas in Guangxi [28]. As detailed in Table 8, the mean As contents in the Fe-Mn nodules with different diameters were 151.33 mg/kg (0.3-0.5 mm), 188.50 mg/kg (0.5-1 mm), and 201.50 mg/kg (1-2 mm) in the study area, which shows that the As content in Fe-Mn nodules increases rapidly with the particle size. Furthermore, we calculated the contribution of the As, \(\text{TF}_{\text{e}}\text{2}\text{O}_{3}\), Mn, \(\text{Al}_{\text{2}}\text{O}_{3}\), and \(\text{SiO}_{\text{2}}\) contents in the Fe-Mn nodules (0.3 mm \(\leq\) diameter \(\leq\) 2 mm) to the host soil, and the results are depicted in Figure 5. The average contribution of As reached 64.45%, with the highest contribution being 82.48%. Additionally, the average contributions of \(\text{TF}_{\text{e}}\text{2}\text{O}_{3}\), Mn, \(\text{Al}_{\text{2}}\text{O}_{3}\), and \(\text{SiO}_{\text{2}}\) were 44.38%, 40.17%, 21.57%, and 7.77%, respectively. These results indicate that, as a part of the soil, Fe-Mn nodules play a significant role in the enrichment of As in karst areas. The XRD analysis showed that the Fe-Mn nodules primarily consisted of quartz, goethite, hematite, lithiophorite, birmesite, and phyllosilicates (kaolinite, illite, and clinochlore) (Figure 6).