Distribution and enrichment characteristics of heavy metals in root and sediments of native and exotic mangrove plants at different tidal levels

To investigate species-specific differences in heavy metal accumulation under varying tidal regimes, the native mangrove Bruguiera sexangula (Lour.) Poiret and the exotic species Sonneratia apetala Buchanan-Hamilton were selected for comparative analysis. Concentrations of Cr, Hg, Cd, and Zn were quantified in roots, rhizosphere sediments, and non-rhizosphere sediments at low (LT), middle (MT), and high (HT) tidal zones. Parameters evaluated included bioconcentration factor (BCF), comprehensive enrichment factor (CEF), root physiological indices (root activity and iron plaque formation), and sediment contamination profiles. Results revealed that sediment characteristics, including heavy metal content, soil organic carbon (SOC), soil organic matter (SOM), and acidity, followed the gradient LT>MT>HT. Sediments surrounding S. apetala exhibited higher overall heavy metal concentrations than those associated with B. sexangula. In B. sexangula, heavy metal concentrations were elevated in rhizosphere sediments relative to non-rhizosphere sediments, whereas the inverse pattern was observed in S. apetala. Across tidal levels, both BCF and CEF values were higher in B. sexangula roots, accompanied by greater root activity and iron plaque deposition, which were positively correlated with sediment heavy metal content. Although neither species functioned as a hyperaccumulator, both demonstrated capacity for root-mediated uptake of heavy metals across tidal zones. Notably, B. sexangula exhibited a more pronounced accumulation potential than S. apetala, highlighting its ecological value for intertidal heavy metal remediation.