Geochemical Distribution Characteristics of Rare Earth Elements in Different Soil Profiles in Mun River Basin, Northeast Thailand

Exploring the distributions of rare earth elements (REEs) in soil profiles is essential to understanding how natural and anthropogenic factors influence the geochemical behaviors of REEs. This study aimed to learn about the distribution characteristics of REEs in soils, including their fractionation and enrichment, and to explore the influence of soil pH and soil organic carbon (SOC) on REEs. One hundred and three samples were collected from six soil profiles under different land uses (paddy field: T1, T3; forest land: T2, T6; wasteland: T4; building site: T5) in the Mun River Basin, Northeast Thailand. The average total REE contents (∑REE) are much lower (<80 mg kg⁻¹) than that of Earth’s crust (153.80 mg kg⁻¹) in soil profiles T2, T3, T4, and T6. The contents of REEs tend to increase slightly with depth in all soil profiles. The ratios of (La/Yb)_N range from 0.35 to 0.96 in most samples, indicating that the enrichment of heavy REEs (HREEs) relative to light REEs (LREEs) is the main fractionation pattern. Samples from profile T2 show relatively obvious negative Ce anomalies (0.55–0.78) and positive Eu anomalies (1.41–1.56), but there are almost no anomalies of Ce and Eu in other soil profiles. Enrichment factors of LREEs (EF_LREEs) range from 0.23 to 1.54 and EF_HREEs range from 0.34 to 2.27, which demonstrates that all soil samples show no LREE enrichment and only parts of samples show minor HREE enrichment. Soil organic carbon (SOC) contents positively correlate with the enrichment factors of REEs (EF_REE) in soil profiles T1 (R = 0.56, p < 0.01) and T6 (R = 0.71), while soil pH values correlate well with EF_REE in soil profiles T2 (R = 0.75) and T4 (R = −0.66, p < 0.01), indicating the important influence of soil pH and SOC on the mobility of REEs in some soil profiles.