National-Regional Joint Engineering Research Center for Soil Pollution Control and Remediation in South China, Guangdong Key Laboratory of Integrated Agro-environmental Pollution Control and Management, Institute of Eco-environmental and Soil Sciences, Guangdong Academy of Sciences, Guangzhou 510650, China.
SCNU Environmental Research Institute, Guangdong Provincial Key Laboratory of Chemical Pollution and Environmental Safety & MOE Key Laboratory of Theoretical Chemistry of Environment, South China Normal University, Guangzhou 510006, China.
Sci Total Environ. 2023 Nov 20;900:166435. doi: 10.1016/j.scitotenv.2023.166435. Epub 2023 Aug 19.
Cd speciation in soil and its transport to rice roots are influenced by the soil pH, oxidation-reduction potential, and mineral transformation; however, the immobilization and migration of Cd in soil-rice systems with different pH values under distinct water regimes remain unclear. This study used Cd isotope fractionation, soil physical analysis, and root gene quantification to elucidate the immobilization and transport of Cd in different soil-rice systems. In drainage soils, the high soil pH enhanced the transformation and magnitude of negative fractionation of Cd from MgCl extract to FeMn oxide-bound pool; however, it favored Cd uptake and root-to-grain transport. Compared with drainage regimes, the flooding regimes shifted fractionation toward heavy isotopes from MgCl-extracted Cd to FeMn oxide-bound Cd in acidic soils (∆Cd = -0.09 ± 0.03 ‰) and to light isotopes from MgCl-extracted Cd to carbonate-bound Cd in neutral and alkaline soils (∆Cd = 0.29-0.40 ‰). The submerged soils facilitated the forming of carbonate and poorly crystalline minerals (such as ferrihydrite), which were transformed into highly crystalline forms (such as goethite). These results demonstrated that the dissolution-precipitation process of iron oxides was essential for controlling soil Cd availability under flooding regimes, and the relative contribution of carbonate minerals to Cd immobilization was promoted by a high soil pH. Flooding regimes induced lower expressions of OsNRAMP1 and OsNRAMP5 to limit the uptake of light Cd isotopes from MgCl-extract pool, whereas a teeter-totter effect on gene expression patterns in roots (including those of OsHMA3 and OsHMA2) limited the transport of heavy Cd isotopes from root to grain. These findings demonstrate that flooding regimes could exert multiple effects on soil Cd immobilization and Cd transport to grain. Moreover, alkaline soil was conducive to forming carbonate minerals to sequester Cd.
土壤中 Cd 的形态及其向水稻根系的迁移受土壤 pH 值、氧化还原电位和矿物转化的影响；然而，在不同水分条件下，不同 pH 值的土壤-水稻系统中 Cd 的固定和迁移仍不清楚。本研究采用 Cd 同位素分馏、土壤物理分析和根系基因定量的方法，阐明了不同土壤-水稻系统中 Cd 的固定和迁移。在排水土壤中，较高的土壤 pH 值促进了 MgCl 提取的 Cd 向 FeMn 氧化物结合态转化，并增强了 Cd 的吸收和根到籽粒的转运。与排水条件相比，淹水条件使酸性土壤中 MgCl 提取的 Cd 向 FeMn 氧化物结合态 Cd 的分馏向重同位素方向移动（∆Cd=-0.09±0.03‰），向中性和碱性土壤中 MgCl 提取的 Cd 向碳酸盐结合态 Cd 的分馏向轻同位素方向移动（∆Cd=0.29-0.40‰）。淹水土壤促进了碳酸盐和非晶质矿物（如针铁矿）的形成，这些矿物转化为结晶度较高的矿物（如赤铁矿）。这些结果表明，在淹水条件下，铁氧化物的溶解-沉淀过程是控制土壤 Cd 有效性的关键，高土壤 pH 值促进了碳酸盐矿物对 Cd 的固定作用。淹水条件诱导 OsNRAMP1 和 OsNRAMP5 的表达水平降低，从而限制了 MgCl 提取池中轻 Cd 同位素的吸收，而根系中基因表达模式（包括 OsHMA3 和 OsHMA2）的跷跷板效应则限制了重 Cd 同位素从根到籽粒的转运。这些发现表明，淹水条件可能对土壤 Cd 的固定和 Cd 向籽粒的迁移产生多种影响。此外，碱性土壤有利于形成碳酸盐矿物来固定 Cd。
