## 3 Results and Discussion
### Temporal Trends in Cd Concentration  
This study focused on analyzing the temporal variations of Cd pollution in Hainan's agricultural soils, incorporating data from studies that involved island-wide sampling, random sampling methodologies, and more than 70 sample points across multiple periods. The analysis identified six key periods (2005, 2009, 2014, 2017, 2019, and 2023) to track changes in Cd pollution trends across Hainan's agricultural soils. The annual sample size ranged from a minimum of 83 to a maximum of 366 (Table 4). Table 4 indicates a fluctuating but overall increasing trend, with Cd peaking between 2017-2023. The increasing rise in Cd concentrations can likely be attributed to the intensification of agricultural activities, including the increased use of fertilizers, pesticides, and herbicides, along with industrial and mining wastewater discharge driven by rapid economic development growth. From 2017 to 2023, both average and maximum Cd concentrations were notably higher than those observed between 2005 and 2014. Specifically, the average Cd level rose from 0.09 mg/kg to 0.16 mg/kg, an increase of approximately 77%. The maximum concentration \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **2005** & **2009** & **2014** & **2017** & **2019** & **2023** \\ \hline Number of Samples & 311 & 280 & 302 & 83 & 133 & 360 \\ Mean (mg/kg) & 0.090 & 0.089 & 0.088 & 0.15 & 0.133 & 0.156 \\ Minimum (mg/kg) & 0.01 & 0.01 & 0.011 & 0.01 & 0.01 & 0.01 \\ Maximum (mg/kg) & 0.64 & 0.64 & 0.64 & 1.63 & 0.548 & 2.3 \\ coefficient & 67\% & 74\% & 70\% & 241.67\% & 78\% & 118\% \\ variation (\%) & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Changes in Cd concentration in agricultural soil on Hainan Island during different time periods (2005–2023). Figure 8: Box plot of soil cadmium assessment index for agricultural land in Hainan Island (Pl, Igeo, ER). also surged from 0.64 mg/kg to 2.63 mg/kg, representing an increase of roughly 3.11 times. The trends in Cd concentration across these two periods differ markedly. From 2005 to 2014, Cd concentrations in Hainan's agricultural soils ranged from 0.01 to 0.64 mg/kg, with a relatively stable mean of 0.09 mg/kg. Although slightly above Hainan's natural background level, these concentrations remained below risk thresholds, indicating relatively safe conditions. However, from 2017 to 2023, the average Cd concentration fluctuated between 0.133 and 0.156 mg/kg, showing an initial decline followed by an increase, with variations from around 11% to 12%. The maximum Cd concentrations during 2017-2023 were 2.5 to 3.5 times higher than those recorded during 2005-2014, with the highest value of 2.3 mg/kg observed in 2023. This concentration is 41 times higher than the natural background value and 7.67 times above the risk screening threshold, indicating severe pollution in some areas. In 2017, maximum concentrations were 29 times the background value. The observed rise in Cd concentrations during 2017-2023 aligns with the intensive use of phosphate fertilizers, which contain Cd levels ranging from 0.1 to 170 mg/kg [46]. Phosphate fertilizers are a primary means of enhancing crop yields in Hainan [31], and their long-term application likely contributes to the progressive Cd accumulation in soils. Alternatively, the accumulation could be caused by the combined effect of tailings and cadmium-contaminated fertilizers [35]. Overall, from 2005 to 2023, the Cd concentrations in Hainan's agricultural soils exhibited a fluctuating yet consistent upward trend in Cd concentrations, with a marked rise during 2017-2023. While the overall average concentration of 0.12 mg/kg remains below the 0.30 mg/kg risk screening threshold (GB15618-2018), the maximum concentration of 3.11 mg/kg exceeds the threshold by 10.37 times, highlighting severe pollution in localized areas.