广西典型酸性火成岩地质低背景区土壤-水稻重金属积累特征及生态风险

为了解地质低背景区农田土壤-作物系统中重金属的富集及迁移特征，采集了广西钦北区878 km2酸性火成岩出露区的30套水稻籽实及根系土样品作为研究对象，研究酸性火成岩地质低背景区土壤和水稻中重金属（As、Cd、Cr、Cu、Hg、Ni、Pb和Zn）分布特征、土壤Cd等重金属的赋存状态及其生物有效性。结果显示：研究区土壤呈酸性，pH值为4.66~5.36；Cd、As、Cr等重金属元素含量较低，均低于或远低于我国土壤环境质量标准规定的风险筛选值，水稻籽实中As、Cr、Hg和Pb均未超标，但Cd超标率为16.7%。土壤中As和Pb主要以残渣态存在，Hg主要以残渣态和强有机结合态存在，而Cd的生物富集系数和生物有效性最高，水溶态、离子交换态和碳酸盐结合态的比例总和为32%。水稻籽实吸收Cd等重金属主要与土壤pH值、Fe、Mn和土壤质地有关。酸性火成岩地质低背景区农田酸性、强酸性土壤Cd等重金属元素含量低，但生物有效性高，其生态风险值得关注。

Transfer characteristics and ecological risk assessment of heavy metals in soil-rice system in typical acid magmatic rock area with low geochemical background of Guangxi

In order to understand the transfer characteristics of heavy metals in farmland soil-crop system with low geochemical background, 30 paired rice grain and soil samples from 878 km2 acid magmatic rock area outcropped in Qinbei district of Guangxi were collected. Specifically, the distribution of heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) in the soils and rice grains, the fractionations and influencing factors on bioavailability of Cd and other metals in the soils were studied. The results showed that soil pH values in studied area ranged from 4.66 to 5.36, which was acidic. The concentrations of Cd, As, Cr and other heavy metals were all lower than or far below the risk screening values specified in Chinese soil environmental quality standard (GB15618-2018). In rice grains from the studied area, As, Cr, Hg and Pb did not exceed the standard; but for Cd an exceedance rate of 16.7% was observed. In soils, As and Pb mainly existed in the residual fraction, Hg mainly existed in the residual and strong organic binding fractions. Moreover, the bioconcentration factor (BCF) and bioavailability of Cd were the highest with 32% being the water soluble, ion exchangeable and carbonate binding fractions. The transfer of heavy metals from soil to rice grains is mainly related to soil pH value, Fe, Mn, and soil texture. Therefore, the ecological risk of heavy metals in farmland in acid magmatic rock area with low geochemical background is worthy of attention due to its acidic/strongly acidic soil with heavy metals being of low levels but high bioavailability.