Environmental Geochemical Behavior of Soil Ferromanganese Nodules: A Review

Ferromanganese nodules (FMNs), secondary products of pedogenic processes, are extensively distributed in karst regions characterized by high geochemical backgrounds and are central to explaining the apparent paradox of high total concentrations but low bioavailability of heavy metals in karst soils. However, there remains a limited understanding of their micro-scale mechanisms, a lack of systematic linkage between the dynamic formation and evolution of nodules and their environmental behavior, and an absence of clear syntheses of the differences and governing patterns of nodule geochemical behavior under varying supergene environments. This study presents a systematic review of recent advances in soil ferromanganese nodule research. (1) The formation mechanisms, evolutionary pathways, and paleoenvironmental implications of nodules are synthesized: physical, chemical, and biological factors collectively govern the systematic growth of nodules, with redox potential serving as the fundamental driver. Based on a comparison of the geochemical characteristics of nodules from different supergene environments, a model for their possible influence pathways is proposed accordingly. (2) The ecological impacts of nodules, their enrichment mechanisms for heavy metals, and associated experimental methodologies are synthesized: FMNs exhibit enrichment factors of one to two orders of magnitude for Cd, Pb, Ni, and other trace elements relative to host soils, yet pH-dependent leaching experiments demonstrate maximum release rates of merely 17% (e.g., for As). The application of techniques such as EXAFS, XANES, and LA-ICP-MS confirms that nodules strongly immobilize heavy metals through surface complexation, specific adsorption, and co-precipitation. Nodules can influence the mobilization and transformation processes of characteristic elements, such as arsenic, chromium, and phosphorus, through oxidation or adsorption. Future research priorities include: establishing dynamic risk assessment and early-warning models based on environmental parameters; enhancing cross-disciplinary integration; incorporating findings regarding nodule environmental effects into soil quality standards and risk management protocols; facilitating precision land management and safe utilization practices; and furnishing scientific foundations for accurate soil risk assessment and sustainable land use in geochemical high-background regions. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202509140239.