土壤-植物体系中锌镉稳定同位素分馏研究进展

Stable isotope fractionation of zinc and cadmium in soil-plant system: A review

稳定同位素分馏技术对于示踪土壤-植物体系中重金属的迁移转化过程具有重要作用.本文阐述了土壤-植物体系中锌镉迁移转化主要涉及的土壤根际过程、根系吸收过程和根部-地上部转运过程及其对应产生的同位素分馏特征.在土壤根际过程,土壤固相对锌、镉的吸附解吸反应影响重金属在土壤溶液中的移动性和同位素组成:锌轻同位素、镉重同位素倾向于被土壤固相释放进入土壤溶液;植物根系活化作用则导致土壤固相结合的锌重同位素的释放.根系吸收过程影响土壤-植物间的同位素分馏:质外体吸附锌重同位素,共质体吸收过程中低亲合力转运系统产生锌轻同位素富集,高亲合力转运系统基本不产生分馏或略产生锌轻同位素富集;植物根系存在含硫基团结合镉,并且仅有低亲合力转运系统对镉轻同位素进行吸收转运.在根部-地上部转运过程,根部区室化作用影响植物体内重金属的迁移和地上部同位素组成:锌重同位素、镉轻同位素倾向于在根部储存,导致锌轻同位素、镉重同位素向地上部迁移.在土壤-植物体系中,锌镉同位素分馏现象存在明显差异,反映出植物对锌镉元素不同的吸收、转运和储存机制.

Stable isotope fractionation provides a useful tool for tracing the transfer of zinc （Zn） and cadmium （Cd） in the soil-plant system. Isotope fracfionation processes include equilibrium ffactionafion, e.g. dissolution, complexation, precipitation, and kinetic fractionafion, e.g. diffusion and evaporation. Therefore, isotope fractionation of metallic element can be used to study the mechanisms of metal translocation in biogeochemical cycling. With the development of multiple collector inductively coupled plasma mass spectrometry （MC-ICP-MS）, Zn and Cd isotopic composition in soil and plant samples can be accurately measured. The determination of Zn and Cd stable isotope ratios usually includes sample digestion, pu- rification and mass discrimination correction. In the soil-plant system, plants are generally enriched with heavy Zn isotopes in comparison to soils, while roots are enriched with heavy Zn isotopes in comparison to shoots. For Cd, shoots are likely to be isotopically heavier than roots. The transport processes of Zn/Cd in the soil-plant system mainly involve transport in rhizosphere, root uptake and transport within plants. The adsorption and desorption of Zn/Cd by soil minerals can affect the metal mobility and the isotope fractionation between soil solid phases and soil solution. Heavy Zn isotopes are likely to be complexed, while light Zn isotopes are more mobile in soil solution. Conversely, soil minerals tend to adsorb light Cd isotopes, thereby resulting in an enrichment with heavy isotopes in the soil solution. Among the rhizospheric processes, the secretion of root exudates and the changes in pH also have an impact on the speciation and translocation of heavy metals, which lead to the release of heavy Zn isotopes into soil solution. Root uptake process is of pivotal importance for the determination of the isotopic composition of the whole plant. The diffusion of Zn2＋ from the soil solution to the root surface leads to a light isotopes enrichment in plant. Heavy Zn isotopes are preferentially chelated or precipitated in the root apoplast. During the transport across the root cell membranes, the low-affinity transport leads to an enrichment with light isotopes in plant, while the high-affinity transport generates little or no isotope fractionation. Specific transporter of Cd in roots has never been found yet. Therefore, Cd may be took up mainly through low-affinity transport systems and thus be enriched in light isotopes in plants. The isotope fractionation between roots and shoots is related to the sequestration of Zn/Cd in roots and the xylem transport processes. The functional groups containing oxygen and nitrogen of organic acids in plant roots tend to chelate heavy Zn isotopes, with subsequent compartmentation in root vacuoles. The light Zn isotopes could be then loaded by the xylem and transferred to the aboveground parts. Interestingly, the intensity of isotope fractionation is mainly related to the transport distance in the xylem. Contrary to Zn, light Cd isotopes mainly bind to the thiol groups in roots, leading to the enrichment in heavy isotopes in shoots. The different be- haviors between Zn and Cd in isotope fractionation in the soil-plant system could reflect the distinct accumulation and tolerance mechanisms of plants in response to Zn or Cd stress.