Multiscale iron cycling in mining-impacted soil-water systems under climate variability: constraints from iron...

Iron (Fe) is a critical redox-active micronutrient element, with its biogeochemical cycling significantly regulating the ecological processes in both terrestrial and aquatic environments. The Fe cycling pathways are profoundly influenced by intensifying anthropogenic activities (e.g., mining and agricultural production). Despite the growing recognition of anthropogenic perturbations to Fe cycling, the underlying mechanisms remain largely unclear. In this context, the Fe isotopic signatures have been employed as a powerful tool for tracing the mobilization and transformations of Fe in various ecosystems. This review primarily examines how the concentration, speciation, and isotopic composition of Fe are altered by the biotic and abiotic reactions that are associated with natural and anthropogenic processes in the terrestrial soil-freshwater systems. We aim to systematically revisit the variations in Fe isotopes across different Fe species, with particular emphasis on the anthropogenic influences on the concentration and isotopic composition of Fe. We highlight mining, metallurgy, agricultural production and urbanization as the key processes driving Fe isotopic fractionation. This review provides broad evidence that anthropogenic activities significantly alter the composition of the Fe pool and its coupled cycling with other elements, and that these changes can be effectively traced using Fe isotopic signatures. Understanding Fe isotopic fractionation enables the quantification of human impacts on Fe cycling and provides practical tools for watershed monitoring, the source tracing of contaminants, and the assessment of ecological remediation.