Strategy matters: Phytoremediation potential of native halophytes is jointly associated with their distinct salt tolerances

Abstract

Halophytes are effective candidates for phytoremediation of heavy metal-contaminated saline soils. Still, little is known about the relationships between their potential for metal enrichment and salt-tolerance strategies. This study comprehensively investigated the plant community and concentrations and accumulation characteristics of heavy metals (As, Cd, Cr, Cu, Pb, and Zn) in the soil and dominant native plants with different salt-tolerance strategy groups, namely salt-dilution (SD), salt-excretion (SE), salt-exclusion (SC), and non-halophyte, in a semiarid region of Northwest China. The results showed that the soil in this area was primarily polluted by As and Cd, which were 5.80 and 21.18 times higher than the corresponding background values, respectively. The primary vegetation was desert steppe, and 95 species of 34 families were investigated, most of which were herbaceous (64.2%) with the Asteraceae dominating. Eleven of the thirteen dominant plants, except Zea mays and Helianthus annuus, were halophytes with distinct salt tolerances, exhibiting differing metal accumulation and transport capacities. All the bioconcentration factors of the selected species for six metals were lower than 1, and their accumulation ability decreased in the order of Cu (0.466) > Cr (0.315) ≈ Cd (0.093) ≈ Zn (0.082) ≈ As (0.076) > Pb (0.049). Salt-tolerance strategies were significantly correlated with As and Cd accumulations in the soil-plant environment, but in diametrically opposite directions. Also, metal accumulations and translocations partly depended on plant salt resistance strategies. Salt-diluting halophytes such as Suaeda glauca and Kochia scoparia seem applicable for phytoextraction of single or mixed metal pollution, particularly Cd. In comparison, salt-excreting halophytes such as Chenopodium glaucum and Salsola collina would be more suited to As phytostabilization due to the potential for second pollution. In conclusion, SD halophytes could be used in various metal-contaminated soils in salt-affected regions, while SE halophytes would be well suited for phytostabilization. This study would enhance the exploitation advantages of halophyte germplasms with different salt tolerances in multiple-stressed environments.