## 2 Materials and Methods
### Data Analysis  
Maps with geographic information were drawn using ArcGIS 10.8. Basic statistical analyses were carried out using SPSS 24.0 and Microsoft Excel 2021. The X-ray diffraction (XRD) pattern was analyzed with Jade 6.0. Other graphics were drawn using the OriginLab Origin v.2021.  
The bioconcentration factor (BCF) is an important index to quantitatively calculate the accumulation of As in plant tissues from the environment [46; 47]. The BCF is calculated as follows:  
\[\text{BCF}=\text{C}_{\text{rice}}/\text{C}_{\text{soil}} \tag{3}\]  
where C\({}_{\text{rice}}\) is the As content in the rice (mg/kg); and C\({}_{\text{soil}}\) is the whole As content in the soil (mg/kg).  
The effect of nodules on soil heavy metal elements is manifested in two ways: the content of the element in the nodules of each grain size and the mass fraction of the element in the nodules of each grain size (i.e., the ratio of the mass of the element in the nodules of each grain size to the total mass of the element in the soil containing the nodules). To investigate the contribution of the nodules to the soil chemical composition and trace  
\begin{table}
\begin{tabular}{c c} \hline
**Extractant** & **Process** \\ \hline Control (HNO\({}_{3}\)) & The sample (100 mg) was placed in a container and mixed with 100 mL of 0.5 mol/L HNO\({}_{3}\). The container was placed at room temperature, stirring for 30 min. \\ \hline Hydroxylamine hydrochloride (NH\({}_{2}\)OH–HCl) & The soil (100 mg) was placed in a container and mixed with 200 mL of 0.1 mol/L NH\({}_{2}\)OH–HCl (non-acidified, pH 3.6). The container was placed at room temperature, stirring for 2 h. \\ \hline Acidified hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)–HNO\({}_{3}\)) & The soil (100 mg) was placed in a container and mixed with 50 mL 30\% of 0.1 mol/L HNO\({}_{3}\). The container was placed at room temperature, stirring for 30 min. \\ \hline Citrate–bicarbonate–dithionite (CBD) & The soil (100 mg) was placed in a container and mixed with a solution containing 40 mL 0.3 mol/L sodium citrate (Na\({}_{3}\)C\({}_{6}\)H\({}_{5}\)O\({}_{7}\)\(\cdot\)2H\({}_{2}\)O), 5 mL 1 mol/L sodium bicarbonate (Na\({}_{2}\)CO\({}_{3}\)), and 1g sodium dithiolite (Na\({}_{2}\)S\({}_{2}\)SO\({}_{3}\)). The container was placed in a 80 \({}^{\circ}\)C water bath and shaken for 1 h. \\ \hline \end{tabular}
\end{table}
Table 2: Detailed steps of the selective extraction procedure [35; 41; 42].  
element content, the parameter PQ (percentage of quality, %) was introduced and calculated as follows [48]:  
\[\text{PQ}=\frac{\sum_{\text{j=1}}^{\text{j}}\text{C}_{\text{i}}\times\text{M}_{ \text{j}}}{\text{C}_{0}\times\text{M}_{0}+\sum_{\text{j=1}}^{\text{j}}\text{C}_ {\text{i}}\times\text{M}_{\text{j}}}\times 100 \tag{4}\]  
where M is the mass of the nodule of particle size j, expressed in g; M\({}_{0}\) is the mass of the soil with the smallest particle size, expressed in g; C\({}_{\text{i}}\) is the content of element i in the nodule of particle size j, expressed in mg/kg; and C\({}_{0}\) is the content of element i in the soil with the smallest particle size, expressed in mg/kg. The greater the value of PQ, the greater the proportion of the element present in the nodule and the greater the nodule's contribution of the element to the soil; the converse trend is observed with the smaller the value of PQ.