Effects of lime and water management on uptake and translocation of cadmium in rice

Abstract: Cadmium (Cd) is a highly toxic element, and can be readily accumulated in crops and thus harm human health by food chain. Agricultural paddy soil contaminated by Cd has become very obvious environmental problem in South China. Rice (Oryza sativa L.) is a kind of important crop and main daily diet and has been considered to be a major source of Cd intake by humans in some parts of southern China. The accumulation of Cd in rice in Cd contaminated paddy field easily exceeds 0.2 mg/kg, which is the limit according to the National Standard of Pollutant in Food of China (GB 2762-2012). In order to reduce the effect of Cd on human body, animal and plant, a field experiment was conducted to evaluate the effect of liming application at rice tillering stage on the distribution and migration characteristics of Cd in organs of rice at the different growth stages under the treatment of intermittent irrigation and flooding in whole growth period. The 4 treatments were designed as follows: 1) Intermittent irrigation treatment, and the plot was irrigated following the local conventional irrigation method to maintain being flooded with 3-5 cm water above the soil surface until the late tillering stage and then was drained, followed by intermittent irrigation 2 times (WCK); 2) intermittent irrigation + lime (WL); 3) flooding in whole cultivation, and the plot was flooded during the whole period of crop growth, in which water was kept at a height of 3-5 cm above the soil surface (FCK); 4) flooding in whole cultivation + lime (FL). Lime (1500 kg/hm2) was applied to the paddy soil after the samples were collected at tillering stage. The samples were collected at stages of tillering, filling and maturity, respectively. The results showed that the intermittent and continuous flooding treatment combined with lime application at rice tillering stage increased significantly the soil pH values and decreased the available Cd mass fraction in the soil. Application of lime caused the transforming of acid-soluble Cd into oxidizable, reducible and residual states. Moreover, application of lime in the treatment of flooding in whole growth period could increase iron (Fe) mass fraction in the soil. After liming, the Cd mass fraction of brown rice significantly reduced from 0.86 to 0.56 mg/kg under intermittent irrigation treatment and from 0.77 to 0.34 mg/kg under the treatment of flooding in whole growth period. Meanwhile lime application reduced significantly the bioconcentration factor of Cd in shoot at filling stage (P<0.05), while the bioconcentration factor of Cd in root and brown rice at maturity stage was also decreased significantly (P<0.05). In addition, at maturity stage the translocation factor from root to shoot and that from shoot to brown rice decreased significantly under the treatment of flooding in whole growth period combined with lime application (P<0.05). The rice yield averagely increased by 10% with lime application. Correlation analysis indicated that Cd mass fraction in brown rice was significantly positively correlated with the available Cd mass fraction in soil and the Cd accumulation in aboveground part of rice, and was significantly negatively correlated with soil pH value. In all, the results indicated that the intermittent irrigation and flooding in whole growth period combined with lime application at tillering stage were effective ways to reduce uptake of Cd in rice in contaminated paddy soil without yield loss, especially the flooding treatment combined with lime application.