Advancing soil science using rare earth element tracing: Review, limitations and perspectives

Rare earth elements (REEs), owing to their unique geochemical properties, have emerged as versatile tracers for investigating soil processes. However, most REE tracing studies remain application-specific, and systematic evaluation of tracing strategies, methodological uncertainties, and future development pathways is still lacking. This review summarizes two fundamental REE tracing strategies, namely active labeling and passive fingerprinting. Active labeling introduces artificial REE tracers to track short‑term soil particle movement. Passive fingerprinting uses inherent REE distributions and anomalies to interpret material sources and soil processes operating over longer periods. The geochemical principles, methodological frameworks, and applications of both strategies in soil formation, environmental source tracing, soil redistribution, aggregate dynamics, and subsurface transport are evaluated. Major uncertainties arise from particle-size-selective adsorption and transport, non-conservative REE behavior, heterogeneous backgrounds, scale mismatch, and bulk digestion signals that integrate reactive and resistant mineral pools. Methodological progress therefore depends on standardized background characterization, minimally disruptive labeling, calibration across particle size fractions, and explicit propagation of uncertainty. Further advances will require portable detection, nested observations across scales, and integration with complementary isotopic proxies. By distinguishing between tracing strategies and matching analytical designs to process scales, this review provides a critical basis for more reliable, comparable, and scale-aware applications of REEs in soil science.