Volcanic soils cover only approximately 1% of the Earth's surface, yet they support 10% of the world's population due to their inherent fertility. However, deep magmatic processes may lead to depletion of certain elements in volcanic rocks. The imbalance of essential elements, such as cobalt, in soil parent materials can affect the health of plants and grazing animals and, thus, humans. Within the particular geological context of the Azores, the present study aims to assess cobalt concentration in volcanic soils to predict the risk of cobalt deficiency in animals and humans. Samples from agricultural topsoils and pasture grass were collected in six volcanic regions of São Miguel Island, and their physicochemical properties were measured, including selected transition metal element contents, such as iron, manganese and cobalt. The soil cobalt concentration was below 5 mg/kg in Povoação< Furnas/Congro< Sete Cidades< Fogo and was higher than 10 mg/kg in Picos< Nordeste. Cobalt concentrations were very low in pasture grass. Values below 0.1 mg/kg were observed in Furnas/Congro<Povoação<Fogo<Picos<Sete Cidades, and only the volcanic region of Nordeste had adequate values of cobalt (0.34 ± 0.19 mg/kg) in pasture grass to fulfill animal needs. The concentrations of iron and manganese were also significantly higher in the two latter volcanic regions. These differences result from the pedogenesis of volcanic rocks with distinct geochemical compositions related to different degrees of magmatic evolution. The results show that there is a significant lack of cobalt in agricultural soils on São Miguel and that it is important to predict the risk of cobalt deprivation due to its significant biological value. Soils deficient in cobalt can reduce plant growth, result in the impairment of vitamin B12 formation in animals, and consequently affect animal and human health.
Keywords: Cobalt; Deficiency; Geology; Health; Risk; Volcanic island.
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