to analyze the concentration, spatial distribution patterns, and ecological risks of heavy metals (Cd, Cr, Pb, As, Cu, Ni and Co), 272 topsoil samples (0-20 cm) were collected from paddy fields in Fujian province in July 2017. The results revealed that the mean concentration of all heavy metals exceeded the background values in Fujian province, with the mean concentration of Cd being 5.20 times higher than its background. However, these concentrations of heavy metals were lower than their corresponding national standards (GB 15618-1995). Spatially, for Cd, the high concentration areas were located mainly in southeast of Sanming city and northeast of Quanzhou city. For Pb and As, the places of highest concentration were mainly in southeast of Quanzhou city and Zhangzhou city, and the main areas of high Ni concentration were distributed southeast of Nanping city. The geo-accumulation index (I geo) of Cd and As were indicative of moderate contaminations, and the index of Co, Cu and Cr suggested that these were practically uncontaminated. The nemerow integrated pollution index (P n) showed that the entire study area was prone to a low level of pollution, but at the county level, Yongcun county and Zhaoan county are in an warning line area of pollution. According to the potential ecological risk (RI), the ecological risk belongs to the low risk of paddy fields in Fujian province. However, Cd should be given attention (E r = 25.09), as it contributed to the majority of potential ecological risks in Fujian province. The large number of industrial waste, mining byproducts, chemical fertilizers, pesticides and other synthetic chemical substances in the agroecological ecosystem has seriously threatened food security, food safety and human health around the world 1-6. Thus, soil pollution treatment is urgently needed to solve the current problems in agricultural production. At the same time, the accumulation of heavy metals in agricultural soil is a serious problem for human health and safe food production 7-9. Heavy metal contaminants are commonly introduced to soils through anthropogenic activities, such as mineral resources development, agrochemical use, fossil fuel consumption, poultry manure, waste disposal, and sewage irrigation 10-12. Soil heavy metal contamination has become serious and widespread in China. According to the national communiqué of the soil pollution survey by the ministry of environmental protection of China and the ministry of land and resources in 2014 13 , some regions of soil have become heavily contaminated, and the quality of cultivated soil is particularly concerning. The total over-standard rate of soil (environmental quality standard for soils in China, GB15618-1995) in China was 16.1%. Further, the over-standard rates of Cd, Cu, Hg, As, Pb, Ni and Cr were 7.0%, 2.1%, 2.7%, 1.5%, 4.8% and 1.1%, respectively 13. A soil pollution control plan has been developed in China to improve soil quality, ensure the quality and safety of agricultural products and protect the health of humans, such as a n...

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A batch experiment was conducted to examine the combined effects of three common low-molecular-weight organic acids (LMWOAs) on the mobilization of arsenic and lead in different types of multi-contaminated soils. The capacity of individual LMWOAs (at a same molar concentration) to mobilize soil-borne As and Pb varied significantly. The combination of the organic acids did not make a marked "additive" effect on the mobilization of the investigated three elements. An "antagonistic" effect on element mobilization was clear in the treatments involving oxalic acid for some soils. The acid strength of a LMWOA did not play an important role in controlling the mobilization of elements. While the mobilization of As and Pb was closely associated with the dissolution of soil-borne Fe, soil properties such as original soil pH, organic matter contents and the total amount of the element relative to the total Fe markedly complicated the mobility of that element. Aging led to continual consumption of proton introduced from addition of LMWOAs and consequently caused dramatic changes in solution-borne Fe, which in turn resulted in change in As and Pb in the soil solution though different elements behaved differently.

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