Effects of organic matter on the distribution of rare earth elements in red soil aggregates during ecological restoration

Rare earth elements (REEs) are widely applied in high-tech fields. However, their increasing presence in the food chain poses significant risks to human health. At present, little is known about the effects of organic matter on the distribution of ion-adsorbed REEs in soil aggregates during ecological restoration. Red soil derived from coarse-grained granite in Southern China is both prone to ecosystem degradation from soil erosion and rich in REEs. Understanding the distribution of REEs in soil aggregates undergoing ecological restoration is helpful to formulate effective measures for controlling the environmental migration of REEs. Four sites that had undergone different durations/degrees of ecological restoration were selected in the areas to analyze. REEs concentration of six different aggregates sizes (<0.25, 0.25–0.5, 0.5–1, 1–2, 2–5, and >5 mm) were analyzed and the enrichment coefficients were calculated in 4 sample sites of severe-degraded ecosystem in Changting County, Fujian Province, Southern China. The results showed that the total rare earth elements (TREEs) concentration in the aggregates increased from 213 mg kg−1 to 528 mg kg−1 with the extension of the ecological restoration time. At the initial stages of ecological restoration, there was no significant difference in the TREEs concentration among the six aggregates sizes. However, in the middle and late stages of restoration, the concentration of TREEs increased significantly with the decrease of aggregate size. The concentration of individual REEs showed three changing trends with sizes of aggregates during ecological restoration, respectively: 1) no obvious regular change (S1), 2) a V-shaped change trend (S2), and 3) increasing concentration with the decrease of aggregate size (S3 and S4). Ce and Eu showed a positive and negative anomaly in the soil aggregates, respectively. Moreover, the light rare earth elements (LREEs) were enriched, while the heavy rare earth elements (HREEs) were depleted during the initial stages, and the HREEs were enriched during the middle and late stages of restoration. The correlation coefficient between organic matter and REEs in aggregates was generally low; however, LREEs showed a stronger correlation with organic matter than that of HREEs during the initial stages of ecological restoration. The correlation between organic matter and HREEs gradually increased and even exceeded that of LREEs with on-going ecological restoration. The distribution of REEs concentration in degraded soil aggregates in Southern China showed obvious variability with the ecological restoration time.