[Distribution and Environmental Significance of Rare Earth Elements in Typical Protected Vegetable Soil, Northern China]

The concentrations of rare earth elements (REEs) in protected vegetable soils in Wuqing district of Tianjin City, Jinzhong district of Shanxi Province, Shenyang district of Liaoning Province, and Wulanchabu district of Inner Mongolia Autonomous Region in northern China were measured to analyze the change characteristics of soil REEs in the process of protected vegetable cultivation. Additionally, we sought to use the REEs parameters to trace the feasibility of characterizing the interference of human activities on the soil ecological environment. The results showed that the total content of REEs (REE) in the topsoil of protected vegetable fields ranged from 146.52 to 158.76 mg·kg^-1, with an average of 152.34 mg·kg^-1 in Shenyang; 92.16 to 137.69 mg·kg^-1, with an average of 115.03 mg·kg^-1in Wuqing; 91.38 to 118.84 mg·kg^-1, with an average of 108.03 mg·kg^-1 in Wulanchabu; and 97.62 to 111.27 mg·kg^-1, with an average of 102.43 mg·kg^-1in Jinzhong. The REEs distribution patterns in the soils of the four areas, standardized with chondrite, characterized by a right tilt, showed that light rare earth elements were obviously enriched in the soil, demonstrated by the ratios of LREE/HREE and (La/Yb) _N, which were greater than 6 and 7, respectively. The values of (La/Sm)_N in the soils were higher than 3, suggesting that there was an obvious fractionation between light rare earth elements, whereas the values of (Gd/Yb)_N were between 1-2, and there was a weak fractionation between heavy rare earth elements. The values of δEu in the soils were between 0.56 and 0.61, showing that Eu had a negative abnormality. The values of δCe were between 0.89 and 1.11, showing that Ce had no abnormality or weak positive abnormality. The higher LREE/HREE and (La/Yb)_N in protected vegetable soil than that in open-air vegetable soil indicated the increasing differentiation degree between light and heavy rare earth elements in protected vegetable soil. The lower (La/Sm)_N in protected vegetable soils indicated the reduction in the differentiation among light rare earth elements in soil. Higher δCe values and lower δEu values suggested that Ce and Eu were relatively enriched and depleted, respectively, during vegetable planting. The REE, LREE, (La/Sm)_N, and δEu in protective soil decreased with the number of cultivation years, whereas the (Gd/Yb)_N and δCe increased, but the HREE values did not change significantly. There was a significant correlation between δCe, δEu, (La/Yb)_N, (Gd/Yb_)N, and soil bulk density, soil moisture content, and soil organic matter in Tianjin protected vegetable soils, showing preliminarily that rare earth elements can be used as tracer elements to characterize the interference intensity of human activities on soil.