Safe Utilization of Paddy Soils Lightly Polluted with Cadmium in Western Chongqing

Abstract

The western Chongqing region is the main grain-producing area in Chongqing. This region's soils are characterized by light-to-moderate cadmium (Cd) pollution. Two types of typical paddy soils in this area (acidic and calcareous purple soils) were selected for the development of safe rice production techniques using in situ field remediation experiments. These involved a low-Cd-accumulating rice variety (Changliangyou 772) grown either alone or in combination with heavy metal passivators (silicon-calcium fertilizer, iron powder, biochar, and straw organic fertilizer). The results showed that:① all of the passivators except for the Fe powder increased rice yields from both types of soils. Straw organic fertilizer showed the best results for the acidic purple paddy soil, with a rice-yield increase of 47.43%, while biochar performed best for the calcareous paddy soil, increasing yields by 23.95%; ② The low-accumulation rice variety alone could not meet the requirements of safe rice production in the acid purple paddy soil (pH=4.75); however, combined the with passivators (with the exception of the silicon-calcium fertilizer), Cd content in rice grains was reduced by 14.81%-54.88% to within the national safe food standard for rice (0.2 mg·kg-1, GB 2762-2017). The Cd content of rice grains varied between 0.012 and 0.030 mg·kg-1 in the calcareous purple paddy soil (pH=7.77), under various treatments, which was far lower than the safety standard. The application of passivators (with the exception of biomass charcoal) further reduced the Cd content of rice by 26.67%-59.00% nevertheless; ③ The use of passivators altered Cd transportation and distribution in the rice plant. Taking the acidic soil as an example, silicon-calcium fertilizer inhibited the transport of Cd from the stems to the rice grains, Fe powder and biochar reduced the enrichment of Cd in roots and inhibited the transport of Cd from the stems to the rice grains, and straw organic fertilizer inhibited the transport of Cd from the roots to the stems; ④ The application of passivators promoted the transformation of soil Cd from labile forms into residual form, reduced the bioavailability of Cd in the soil, and, thus, reduced the accumulation of Cd in the rice plants. In the acidic soil, biochar showed the best effect, while straw organic fertilizer performed best in calcareous soils. ⑤ Silicon-calcium fertilizer and straw organic fertilizer significantly increased the pH and organic matter content of acid soils. Consequently, the soil available Cd content decreased by 39.45% and 34.69%, respectively, while no such effects were observed for the calcareous soil.