[Translocation, Accumulation, and Comprehensive Risk Assessment of Heavy Metals in Soil-Crop Systems in an Old Industrial City, Shizuishan, Ningxia, Northwest China].

In order to explore the environmental geochemistry characteristics of heavy metals (HMs) in soil-crop systems in an old industrial city, the concentration and fraction of HMs in the paddy, wheat, and maize root soil and their seeds were detected and analyzed. Subsequently, statistical methods, risk assessment coding (RAC), the bio-enrichment coefficient factor (BCF), influence index of comprehensive quality (IICQ), and ArcGIS spatial interpolation were used to conduct the translocation, accumulation, and comprehensive risk assessment of HMs in soil-crop systems. The results showed that the average concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in root soil were ranked respectively as follows:12.56, 0.19, 63.48, 23.52, 0.038, 28.86, 21.68, and 69.47 mg·kg-1. HMs in root soil were accumulated to some extent in comparison with the soil background value in Ningxia, especially Cd and Hg, but did not exceed the soil environmental pollution screening value (GB 14618-2018). The average concentrations of the eight aforementioned elements in supporting crop seeds were 0.0149, 0.0112, 0.075, 6.7, 0.0015, 0.67, 0.0427, and 20.48 mg·kg-1 in turn. The over-limit ratio of As, Pb, and Cr in crop seeds was 4%, 3%, and 1%, respectively, relative to the national food safety standards (GB 2762-2017), whereas the other five elements were within the allowable range. In comparison to those in paddy and wheat, HMs hardly tended to translocate to maize seeds from root soil. According to the results of IICQ in soil-crop systems, the cultivated soil was in the state of slight sub-contamination regionally, and only 10% of sampling points showed slight (sub-)contamination-submoderate contamination, where we could replant maize to reduce HMs contamination risk.

[Threshold of Se-rich Soil Based on Available-Se and Influencing Factors of Available-Se].

Available selenium (Se) in soil was the predominant factor affecting the content of Se in crops. In order to reasonably delineate the Se-rich soil range and propose theoretical guidance for the cultivation of natural Se-rich crops in a region where the surface soils had a high level of available-Se and a low level of total-Se, 8814 samples in surface soil and 195 root-crop matching samples were collected in Shizuishan in northern Ningxia. On the basis of the main line of analysis of available-Se, the following research was conducted: by synthetically studying the total-Se and available-Se in surface soil and root soil, the morphology of Se in surface soil, as well as Se in crops, deep and coordinated analyses of content among total-Se, available-Se, and Se in root-crop matching samples were carried out, and the suitable threshold for Shizuishan was confirmed. A multiple regression model of available-Se was established to determine the main physical and chemical indexes affecting available-Se, which were expected to improve the Se enrichment rate of crops through the enhancement of available-Se. The results demonstrated that ω(Se) and ω(Seavailable)in the surface soil in Shizuishan were 0.26 mg·kg-1 and 12.85 µg·kg-1, respectively, and the characteristics of Se and available-Se in root-crop matching samples could represent those in surface soil. Thus, it was recommended to use 0.24 mg·kg-1 as the suitable threshold of Se-rich soil. The multiple regression model of available-Se showed that the increase in total-Se and soil elements affecting soil fertility could promote the enrichment of available-Se.

[Enrichment Factors of Soil-Se in the Farmland in Shizuishan City, Ningxia].

Shizuishan is a typical exhausted resources-based city in the northern area of the Ningxia Hui autonomous region in China. In order to develop the planting industry of selenium (Se)-rich agricultural products and promote green and sustainable urban development and transformation, investigations on the quality of Se-rich land were carried out in Shizuishan City, where 7399 surface soil (0-20 cm) samples of farmlands, 30 atmospheric precipitation samples, and nine parent rocks were collected. By means of semi variogram model construction by GS+, Kriging interpolation in ArcGIS and statistics via SPSS, such as correlation analysis and mean-value analysis, the content, spatial distribution, and enrichment factors of Se-soil were analyzed. Further, the enrichment characteristics of soil Se in alkaline conditions were summarized. The results indicated that ω(Se) in surface soil was (0.26±0.12) mg·kg-1, and its spatial distribution was highly auto-correlated. The variation in Se content was related to natural factors. Along Helan Mountain, the content of Se in the surface soil was comparatively higher than that where coal mines were located. The parent rock was the principal factor that controlled the enrichment of soil Se. The physical and chemical properties of soil such as organic material, pH, and iron and manganese oxides had crucial effects on the enrichment of soil Se in a surficial environment. Compared to a strong alkaline environment, alkaline conditions were beneficial for the enrichment of Se in the surface soil.