In recent years, soil pollution by heavy metals in China has become a critical issue. Heavy metal pollution poses threats to the local environment, food safety, and human health. Compared with water and air pollution, soil pollution by heavy metals is invisible, has poor self-purification ability, and has high associated risks due to accumulation. Heavy metal pollution also increases the risk of cancer in children and residents living near heavily polluted areas (Fan et al. 2013; Li et al. 2014). Pollution risk assessments, migration simulations, source analyses, and remediation of heavy metal pollutants in farmland soils have been conducted to address widespread concerns (Gu et al. 2014; Xu et al. 2017; Zhao et al. 2018; Qiao et al. 2019). Approximately 2 × 107 hm2 of cultivated land is affected by heavy metal pollution in China, while as much as 1.2 × 107 t of grain is contaminated annually, with an economic loss of 2 × 1010 yuan (Wu et al. 2010). In the past 20 years, research on heavy metal pollution in the soil has indicated that heavy metal pollution levels differ depending on the area, such as farmland, cities, suburbs, and rural regions. In China, 83.9% of the provinces and 22.5% of the prefecture-level cities are affected by heavy metal pollution (Song et al. 2013). The pollution of regional farmland soil by heavy metals is an important issue, which is especially prominent in regions such as southwest China (Yunnan, Guizhou), central China (Hunan, Jiangxi), the Yangtze River Delta, and the Pearl River Delta (Zhao and Luo 2015). In North China, the main source of heavy metal pollution sources in farmland soil is atmospheric subsidence, while the sources in South China are mainly agriculture and livestock production (Peng et al. 2019). Imperfect remediation technology and a shortage of long-term risk control mechanisms for restoration measures are major challenges for effectively preventing and controlling heavy metal pollution in farmland soil (Chen et al. 2018). In May 2016, the State Council issued the Soil Pollution Prevention Action Plan (referred to as the "Ten Articles"), which reflect the importance of preventing and controlling heavy metal pollution in soil. It also strengthened the current regional treatment and remediation practices (Luo and Teng 2018).
Heavy metal migration is part of the solute transport system in soil. Two common methods used to analyze heavy metal migration in soil are column leaching and natural leaching tests (Yang 2017; Zhang et al. 2018). In recent years, column leaching (filtration) has become a commonly applied method for analyzing heavy metal migration and accumulation. Current research includes analyses of the migration rate and morphological compositions of heavy metals, as well as exploring the migration characteristics of different heavy metals under different conditions (Shangguan et al. 2015; Li and Wu 2017). Under natural conditions, the vertical distribution and migration of soil elements in a profile are affected by the chemical and physical properties of the soil (Ye et al. 2012; Ye et al. 2016). In general, the heavy metal contents in soil colloids are much higher than what is observed in coarse soils. The distribution of heavy metals in soils is affected by the composition of organic matter, iron/aluminum oxides, and clay minerals in the soil (Liu et al. 2018). Organic matter in the soil, particularly humic and fulvic acid, has a high adsorption capacity for many pollutants, including heavy metals. This can reduce the heavy metal absorption by plants, fix heavy metals in the soil, and reduce the migration of heavy metals into the groundwater (Jolanta 2018). The pH affects the presence of various elements in the soil and determines their migration, thereby affecting the migration, enrichment, and transformation of heavy metals (Chen et al. 2016; Yang et al. 2017). Currently, there is little research on heavy metal leaching characteristics under natural conditions, and available research methods are limited. Compared to column leaching, which is performed in the laboratory, natural leaching tests have conditions that are similar to the natural environment and can be performed at larger scales (Shangguan et al. 2015). Therefore, this study used column leaching and natural leaching to compare the heavy metal leaching and release characteristics at different soil depths under simulated rainfall conditions. We also clarified the accumulation and migration mechanisms of heavy metals at different depth, and provide a theoretical basis for improving and remediating the soil after heavy metal pollution.