Pedogenic ferromanganese nodules and their impacts on nutrient cycles and heavy metal sequestration

Ferromanganese nodules (FMNs) are common products of pedogenesis and differ significantly from the surrounding soil matrix in terms of morphology, microstructure, and chemical composition. FMNs have direct impacts on nutrient cycles and heavy metal adsorption, and the concentric ring structure (or undifferentiated fabric) of FMNs can serve as fingerprints in assessing the paleoclimatic conditions and pedogenic environments. Despite the extensive research on the formation, characteristics and environmental implications of FMNs, little is done in terms of review highlights of the research achievements, gaps and future directions in this field. This paper reviewed and summarized the advances and achievements in the research of pedogenic FMNs, including i) formation mechanisms, evolution processes, and influencing factors of FMNs; ii) variations in micromorphology , elemental contents, and mineral compositions of FMNs in different soil types; and iii) sorption , substitution, and fixation of soil nutrients and heavy metals by FMNs. The areas of future studies included: i) determining the age, formation rate, and environmental threshold of FMNs at different stages of pedogenesis using numerical dating techniques; ii) establishing FMN evolution models for both natural and anthropogenic soils to better understand the complex changes of FMNs; and iii) clarifying the processes and mechanisms governing the fixation and re-activation of soil nutrients and heavy metals by FMNs via combined controlled experiments and isotope techniques . Addressing the above areas requires multidisciplinary collaborations, multi-analytical techniques and advanced isotope chronology. Clearly, the potential exists to vastly improve mechanistic understanding of FMN formation and evolution and the ways in which this can influence biogeochemical cycles .