Unraveling the pathways of heavy metal accumulation in rice: roles of roots, stems, and soil pH

Heavy metal contamination in agricultural soils is a global concern, yet the roles of roots, stems, and soil pH in metal uptake by rice remain underexplored. This study evaluates the ecological and health risks of soil and rice contamination and examines how metal levels in soil, roots, stems, and soil pH affect accumulation in rice grains. A total of 520 paired soil-grain samples and 40 comprehensive sets including soil, rice roots, stems, and grains were collected and analyzed. The results indicated that the ecological and health risks associated with heavy metals were generally low. However, accumulation in rice roots and stems may contribute to secondary soil contamination. Grain Hg and Cu concentrations showed positive, non-linear associations with their respective soil levels. Cd in grains was positively and non-linearly associated with root concentrations, while Cu showed a positive linear relationship with root levels and a negative linear relationship with stem levels, suggesting that stems may act as a barrier to Cu translocation. Cr in grains was positively and non-linearly associated with stem concentrations. Soil pH significantly influenced uptake: Cd absorption was enhanced under acidic conditions (pH ≤ 5.5) and inhibited under alkaline conditions (pH > 7.5); Hg uptake increased at pH > 6.5; and Cu and Cr uptake were higher at pH > 7.5. This study fills key knowledge gaps by clarifying the roles of roots and stems in metal translocation and offers insights into mitigating heavy metal accumulation in rice through soil pH management.