Assessing the ecophysiological effects of iron mining tailings on velvet bean: implications for growth limitations based on mineral composition and physicochemical properties of tailings-soil substrates

Besides the human need for mining activities, they contribute to environmental degradation in different ways, such as physical destruction of the environment, emission of dust containing toxic elements into the air, contamination of soils and aquatic environments by toxic elements, and dam collapses. In November 2015, the largest world’s iron mining disaster occurred in Mariana (Minas Gerais state, Brazil) and the environmental damage to the Atlantic Rainforest in that region was inestimable. Both Fe and Mn are commonly found in the same ore, and the Fe mining process extracts most of the Fe and discards all the Mn. In a way, even if Mn is considered inert in terms of health risk to plants and animals, high contents of this element, such as those observed in studies on the tailings of the Fundão dam, are of concern. There are even studies reporting that high concentrations of this plant micronutrient may cause physiological disorders in plants, which can further impact the recovery process in areas contaminated by tailings, using selected exotic species or naturally by native ones. We aimed to evaluate ecophysiological changes and growth-limiting factors in the native species Mucuna pruriens var. utilis (Wall. ex Wight) Baker ex Burck grown on tailings. Five new growth substrates were prepared by mixing different volumes of pure iron ore tailings from the Fundão dam with a fertile soil-substrate: 0 (control, fertile substrate), 25, 50, 75, and 100% tailings. We evaluated different physicochemical properties and mineral composition of the growth substrates. In plants, we also evaluated mineral composition and physiological traits as gas exchange, chlorophyll a fluorescence, carbohydrate and proline contents, dry mass, and superoxide dismutase activity. The most contrasting results were observed comparing the treatment control to the treatment with pure iron ore tailings, with a trend of intermediary results for the other treatments (25–75% tailings). The higher the tailings concentration, the more impaired was the growth. More than just to low tailings fertility, these results were related to the high concentrations of Mn found in substrates and plant leaf tissues. We also observed that photosynthesis was impaired through disturbances in photochemical and gas exchange parameters probably due to the high levels of Mn and the associate ionic imbalance it causes to the Fe/Mn ratio. Thus, more attention should be paid to Mn as an important environmental contaminant and new studies focused on how tailings (and probably Mn toxicity) may affect plant growth should also include histological, metabolic, and molecular approaches, as well as relating the assimilation and contents of Fe and Mn in roots, stem, and leaves.