Ecotoxicological risk of cerium for tropical soils

Abstract

Cerium (Ce) is present in modern industry and in the composition of phosphate fertilizers. The discharge of high technology materials and intensive application of phosphate fertilizer, as it happens in tropical soils - e.g., soils of Brazilian farming areas - may change the natural status of Ce in the soil environment. Thus, understanding the environmental consequences of Ceimput in tropical agroecosystems is important to evaluate the potential ecological risk of this element and ensuring environmental safety. The aims of this study were to evaluate the ecotoxicological risk to plants growing in tropical soils contaminated with Ce, as well as to create a database to support future legislation regulating the limits of this element in Brazilian and conceivably other tropical soils. In addition, this study evaluated the effects of Ce on physiological processes of plants, as well as suggested plant physiological variables for assessing ecological risks for this element. Eight crop species (corn, sorghum, rice, wheat, soybeans, sunflower, radish, and beans) were exposed to a Ce concentration gradient in two typical tropical soils (Oxisol and Inceptsol), and an artificial soil. Our findings showed that among the growth and germination endpoints measured, Cephytotoxicitywas more pronounced on shoot dry matter than on percent germination and germination speed index, irrespectively of the soils and crop species evaluated. Sensitivity of plants is species specific. Soil properties such as pH, cation exchange capacity, and organic carbon may have influenced the severity of Cephytotoxicity. Because of that, Ce was more toxic to plants in the Oxisol than the other soils tested (Inceptsol and artificial soil). Our risk assessment results (hazardous concentration, HC5 = 281.6 mg Ce kg-1) support the idea that unintentional Ce input through P fertilizers does not pose a risk to soils of Brazilian agroecosystems. Among the physiological variables measured, Cephytotoxicitywas more pronounced on SPAD index than on photosynthetic rate, transpiration rate, and stomatal conductance. Our risk assessment results with physiological variables support the idea that SPAD index may be a variable used in ecological risk studies for Ce, because it is both very sensitive and easy to measure.