Lanthanum ecotoxicology and effects on crop species grown in tropical soils

Abstract

The rare earth elements (REE), among which lanthanum (La), are essential for high-tech industries due to their unique properties as malleability and ductility. Thus, the concentrations of those elements in the environment have increased in recent decades due to industrial waste disposal and the application of phosphate fertilizers, since these could contain significant concentrations of REE. Therefore, it is necessary to assess the risks that the organisms are subject to in an environment with concentrations of REE above natural ones, mainly in agricultural areas requiring high applications of phosphate fertilizers, as is the case of tropical soils. In this sense, we aim in this study: (i) to evaluate the ecotoxicological risk of La and to propose a limit value of the addition of the element in tropical soils to prevent phytotoxicity (ii) to evaluate the La effect on the nutrient contents in the shoot of plant species grown in tropical soils. Ecotoxicological tests following standard protocols were carried out in which four plant species (maize, sorghum, soybean, and sunflower) were exposed to increasing concentrations of La applied in three soils, two natural soils (Dystrophic Red Oxisol - LVd and Dystrophic Red-Yellow Oxisol - LVAd) and the other a tropical artificial soil (SAT). The results varied according to the plant species and the soil, evincing how the physicochemical properties of the soil determine the effects of La on plants and how these present different responses to the element. The harmful effects were more pronounced in plants grown in the natural soils than in SAT, mainly in the LVd, for which the lowest toxic endpoints were found. Also, for the shoot of the plants grown in these soils, it was observed, in general, the higher occurrence of reduction of nutrient contents and an increase of La contents. On the other hand, for plants grown in SAT, less phytotoxicity of La was observed, which in general occurred from 614 mg La kg-1 of dry soil. Moreover, in general, the nutrients' levels in the shoot of those plants increased or did not show significant effects concerning the controls. Also, in the SAT, the phenomenon known as hormesis was observed, mainly for maize plants, which presented slight stimulating effects on the shoot dry matter and photosynthesis. In general, considering all soils and species, the sunflower was the most sensitive species to La. Furthermore, the shoot dry matter was the most affected biological endpoint, except for maize grown in the LVd because, for this specie, the photosynthesis showed the lowest values of toxic endpoints. Based on the EC20 values (concentration of La that caused 20% of effect on biological endpoint) found for plants grown in the natural soils, the La limit value of 147.6 mg kg-1 of dry soil was proposed, a preventive value for phytotoxicity and possibly ecotoxicity in the studied soils.