Cobalt in the soil-plant system across deficient, beneficial and toxic levels: data analysis and mechanistic interpretations

Cobalt (Co) is a transition metal essential for human and animal health. Cobalt is classified as a beneficial element for plants, but its precise physiological roles in plant metabolism remain enigmatic. Despite the significant projected 200-500% increase in the industrial application of Co, there is limited literature available on the role of Co in the soil-plant-human continuum compared with other heavy metal(loid)s. Notwithstanding its beneficial roles, Co can negatively affect physiochemical processes in plants both at higher (toxic) and lower (deficient) levels of application. High concentrations of Co cause irreversible changes to plant cells, primarily through the enhanced production of reactive radicals. Similarly, Co deficiency inhibits certain essential plant physiological/biochemical processes. While the optimum levels of Co regulate numerous metabolic and developmental traits of plants. Henceforth, monitoring and understanding the dynamics of Co across deficient, beneficial, and toxic levels is imperative. This review presents a data analysis of the latest literature on Co, including (i) levels and sources in soil, (ii) mobility and phyto-availability, (iii) phytouptake and translocation, (iv) toxic, deficient, and beneficial effects, (v) plant tolerance mechanisms, and (vi) role under environmental stresses. A literature data analysis of 1681 plant observations revealed that plant responses vary significantly for different applied conditions and levels, plant species, and physiological attributes. Overall, the current review provides an updated and critical representation and mechanistic interpretation of the biogeochemical behavior of Co in soil-plant-human systems.