Lime based amendments inhibiting uptake of cadmium in rice planted in contaminated soils

Abstract: Contamination of heavy metals in agricultural soil has been a worldwide challenge for the food security and the health. Especially, cadmium contamination in soil, it is a serious problem in south China. Therefore, it is imperative to develop remediation techniques which are able to stabilize contaminants in situ in an efficient and cost effective manner. Lime is widely employed as a heavy metal immobilizing agent in soil, however, using excess amount of lime can lead to alkaline and compacted soil. In this regards, lime based amendments were considered as an effective remediation method of heavy metals in soils. A field experiment was conducted to evaluate the effects of lime based amendments on cadmium uptake and accumulation in rice (Oryza sativa L) in paddy soil under different Cd levels. The treatments were designed as follows: CK, lime (L), lime + sepiolite (LS), lime + organic fertilizer (LO), lime + calcium magnesium phosphate fertilizer (LP), lime+ calcium magnesium phosphate fertilizer + organic fertilizer (LPO), lime+ sepiolite +organic fertilizer (LSO). The amendments were mixed with the soils by plowing repeatedly, then the soils were equilibrated for 7 days under a maintained soil moisture. The rice cultivar (Oryza sativa L.) was transplanted to soils on April, 2016. Three guard rows were set up around each plot to prevent rice cross-pollination between adjacent treatments. Rice samples from each treatment plot were collected at harvest time (July, 2016). The results showed that the soil pH value were increased significantly after the application of lime based amendments in the field, and the contents of Cd in rice root, stem and brown rice were reduced significantly. Compared with the control plot, the average pH value of soils raised 1.08, 0.96 and 0.93 units respectively under LS, LO and LSO treatments, and the acid extraction Cd contents in soils were reduced by 20.6%, 15.6% and 21.2%, respectively under LS, LO and LSO treatments. The Cd contents in brown rice were lower than the National Standard of Pollutant in Food of China (GB 2762-2012) under L, LS, LO and LPO treatments in paddy soils under all Cd pollution levels. The brown rice Cd content was 0.119, 0.122 and 0.151 mg/kg and reduced significantly by 48.3%, 46.7% and 34.2%, respectively, compared to the control, in mild Cd contaminated soils in LS, LP and LO treatment. With lime mixed with organic fertilizer, calcium-magnesia phosphate fertilizer and calcium-magnesia phosphate fertilizer and organic fertilizer application, the brown rice Cd content was 0.113, 0.125 and 0.150 mg/kg and reduced significantly by 52.8%, 47.8% and 37.5%, respectively, compared to the control, in the moderate Cd contaminated soil. Likewise, in heavy contaminated soil, the brown rice Cd content was 0.156, 0.177 and 0.184 mg/kg and reduced significantly by 51.2%, 44.6% and 42.5%, respectively, compared to the control, with lime mixed with calcium-magnesia phosphate fertilizer and organic fertilizer, organic fertilizer and sepiolite application. Correlation analysis showed that the total Cd contents and acid extraction Cd contents in the soils were positively correlated with the Cd contents in rice root, stem leaf and brown rice (P<0.05). The results showed that the application of LS, LO and LPO treatments can meet the food safety standards in the Cd pollution paddy soils.