Abstract:

[Objective] Rare earth elements (REEs) are key strategic resources supporting the development of high-tech fields, including modern new materials, new energy, electronic information, and national defense science and technology. The spatial distribution patterns and driving mechanisms in the soil environment are critical to the exploration of rare earth resources, the maintenance of regional ecological and environmental security, and the long-term regional sustainable development. South China is the primary mineralization and enrichment zone of ion-adsorption rare earth deposits in China. The warm and humid subtropical monsoon climate, intense chemical weathering, and widely distributed granite weathering crust provide favorable conditions for the activation, enrichment, and fractionation of rare earth elements. Wuhua Country in Meizhou City, Guangdong Province, a typical area ion-adsorption rare earth elements enrichment area in South China. It is critical to systematically reveal the geochemical characteristics, spatial distribution pattern, and multi-scale influencing mechanisms of soil rare earth elements in such a high-REE background area. [Methods] To achieve this goal, a total of 180 topsoil samples, including 27 garden soil samples mainly for fruit and vegetable cultivation, 34 paddy field samples, and 119 forestland samples were collected and the contents of 14 rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were determined in the present study. The enrichment characteristics, light-heavy rare earth fractionation patterns, and spatial distribution patterns of these 14 REEs in the surface soil of the study area were systematically clarified by comprehensively using descriptive statistical analysis and inverse distance weighting spatial interpolation. Furthermore, the influencing of climatic factors, topographic factors, and landscape heterogeneity factors on the spatial differentiation of soil rare earth elements were analyzed at three spatial scales, i.e., 2 km×2 km, 5 km×5 km, and 10 km×10 km. In addition, the coupling effects and scale differences between natural processes and human activities were identified. [Results] The results showed that all 14 rare earth elements at all sampling sites in the study area exhibited significant enrichment. The average total content of rare earth elements (REEs) reached 320 mg·kg⁻¹, which was much higher than the soil REE background values of both Guangdong Province, China and the world. The fractionation between light and heavy rare earth elements was obvious, with light rare earth elements (LREEs) were relatively enriched and heavy rare earth elements (HREEs) relatively depleted. The mean LREEs/HREEs ratio was 14.98, significantly higher than the regional and global averages. This phenomenon could be ascribed to the fact that Wuhua County has a subtropical monsoon climate with frequent and abundant precipitation and active water infiltration and runoff. In general, HREEs were more likely to form bicarbonate and organic complexes and be removed by running water, whereas LREEs tended to be adsorbed by clay minerals, leading to the fractionation between light and heavy rare earth elements. In terms of the spatial distribution of soil rare earth element contents, the high-value areas of REEs and LREEs were concentrated in the northern and western low-altitude regions of the study area, while the high-value areas of HREEs were mainly distributed in the northeastern part, and high-value areas of light-heavy rare earth fractionation were located in the northwestern corner. The overall spatial pattern was significantly coupled with topography: rare earth contents were relatively low in high-altitude and steep-slope areas, while low-altitude and gentle areas were more conducive to enrichment. Multi-scale correlation analysis indicated that the spatial heterogeneity of soil rare earth elements in the study area was jointly regulated by natural factors and anthropogenic activities, showing a significant scale effect. At the large scale of 10 km×10 km, natural factors such as climate and topography dominated the spatial distribution pattern of rare earth elements. The average annual temperature was significantly positively correlated with REEs, LREEs and HREEs, probably suggesting that high temperature promotes rock weathering as well as the activation and enrichment of rare earth elements. The average annual precipitation was significantly negatively correlated with the contents of the three types of rare earth elements, which can be attributed to the accelerated migration and loss of rare earth elements caused by heavy rainfall. Topographic complexity indicators such as altitude, slope and surface relief were generally negatively correlated with rare earth element contents, suggesting that steep terrain tended to promote the migration of rare earth elements with water and soil, while low-lying and gentle areas were more favorable for enrichment. With the decrease of spatial scale, the influence intensity of natural factors gradually weakened and the regulatory effect of anthropogenic activities gradually strengthened. At the small scale of 2 km×2 km, landscape heterogeneity factors clearly reflected the impact of human activities. Indicators representing the intensity of human activities, including paddy field proportion, edge density and area-weighted mean patch fractal dimension, were significantly positively correlated with the contents of REEs, LREEs and HREEs, and negatively correlated with the LREEs/HREEs ratio. Indicators reflecting the integrity of natural landscape, such as forestland proportion and mean patch size, were negatively correlated with rare earth element contents and positively correlated with the fractionation of light and heavy rare earth elements. This demonstrated that anthropogenic activities intensified the enrichment of rare earth elements in surface soil and weakened the fractionation characteristic of relative depletion of heavy rare earth elements to a certain extent. In addition, road density was insignificantly correlated with rare earth element contents and fractionation at all three scales, indicating that traffic emissions contributed weakly to rare earth enrichment in the study area. [Conclusion] In summary, the spatial distribution of soil rare earth elements in Wuhua County was the comprehensive result of the multi-scale coupling of natural processes and anthropogenic activities. At large scales, the macro-pattern was dominated by climatic weathering and topography-driven water-soil migration, while at small scales, it was disturbed by anthropogenic activities such as land use and landscape fragmentation, which locally altered the characteristics of rare earth enrichment and fractionation. The findings of this study could provide important data support and scientific theoretical basis for soil environmental risk prevention and control, rational development of rare earth resources, sustainable land use, and ecological environment protection in high-abundance rare earth regions of South China.

Key words: Wuhua County, rare earth elements, spatial distribution, influencing factors, multi-scale analysis

摘要：

【目的】以广东省梅州市五华县这一典型稀土高丰度区为研究对象，调查表层土壤稀土元素含量水平及空间分布特征，并探讨不同空间尺度下气候、地形和景观异质性因子对其空间分异的影响机制。【方法】采集180个表层土壤样品，测定14种稀土元素含量，运用统计和空间插值方法，分析稀土元素富集特征、轻重稀土分异及空间分布；从2 km、5 km和10 km等3个空间尺度，分析气候、地形及景观异质因子与稀土元素含量的相关性，识别自然因素与人类活动的耦合效应及其尺度效应。【结果】研究区表层土壤中14种稀土元素均呈显著富集，REEs平均值为317.83 mg·kg⁻¹，显著高于广东省、中国及世界土壤背景值。轻重稀土分异明显，LREEs/HREEs均值为14.98，表现为轻稀土相对富集、重稀土相对亏损。空间分布上，REEs和LREEs高值区集中于北部和西部低海拔区域，HREEs高值区主要分布在东北部。多尺度分析表明，10 km大尺度下，年均气温与稀土含量呈显著正相关，年均降水量及地形复杂度指标与之呈显著负相关，自然因素主导宏观格局；2 km小尺度上，水田占比、边缘密度等人类活动强度指标与稀土含量呈显著正相关、与轻重稀土分异呈负相关，而林地占比、平均斑块面积表征自然景观完整性的指标与之相反，人类活动在小尺度上加剧了稀土富集并削弱了重稀土亏损特征。道路密度在各尺度与稀土元素含量及轻重稀土分异均无显著相关性。【结论】五华县表层土壤稀土元素空间异质性是由自然过程与人为活动多尺度耦合作用的结果，交通源贡献不显著。

关键词: 五华县, 土壤稀土元素含量, 空间分布, 影响因素, 多尺度分析