Potentially toxic elements from phosphate fertilizers to crops: mobilization and mitigation

Abstract

Phosphorus (P) is one of the required elements for plant growth and development. So, adequate inputs of P in the soil are essential for food production. However, phosphate rock may naturally contain different levels of potentially toxic elements (PTEs), like arsenic (As), cadmium (Cd), and chromium (Cr). Among the three selected elements, Cd is the most easily bioavailable to the plants, and so food chain. So, this element has been a stepping-stone toward achieving sustainable and safe worldwide food production, especially after a new European Union regulation aiming for reduced limits of Cd in P fertilizers (EU 2019/1009). Soil pH increase is one of the effective strategies to cope better with Cd in soil, as well as mineral supplemental with antagonist elements to Cd, like selenium (Se). In this context, this study aimed to determine the As, Cd, and Cr levels in three P fertilizers (MAP1, MAP2, and MAP3) with contrasting PTEs levels. It also investigated the accumulation and impacts of these elements in the soil and relevant crops known for their contrasting tolerance to these metal(loid)s (potatoes, tobacco, rice) grown in tropical soil, with and without liming. It also investigated the Cd-Se interaction and metabolic adjustments to minimize the plant physiology (Arabidopsis and lettuce) disturbance due to Cd uptake. A diverse array of sensitive techniques for trace elements determination were used to reveal the highest level of Cd of MAP 3. MAP 3 loaded the highest amounts of this element to the soil matrix and solution, as well as to plant shoots and the potato and tobacco xylem sap, contrasting with results for MAP 1. The higher level of Cd in MAP 3 reduced Mn and Zn levels in plant shoots for potatoes and tobacco while also inducing an increase of antioxidant enzymes and sugars in rice. Soil acidity increased Cd concentrations in soil and plants decreased plant growth, yield, and sugar content, and induced metabolic adjustments related to antioxidant enzymes and proline. Among the studied crops, rice had a lower Cd translocation – root-to-shoot transport - compared with potato and tobacco and a higher tolerance to acidic pH. Regarding Se, this element was efficient in increasing root growth, improving photosynthetic performance, stimulating S/Se metabolism compounds, and significantly increasing the content of S, Mo, and Fe, drastically reduced by Cd stress. Our findings highlight the use of low Cd fertilizers to reduce soil contamination risks, as well as the relevance of soil management and mineral supplementation aiming optimize the growth and development of plants exposed to these elements.