The mechanism and technical model of constructing a "loamy-sandy-clayey" soil profile configuration using iron tailings

To address the problems of large-scale accumulation of iron tailings and other wastes generated by a large number of iron ore mines in the semi-arid ecologically fragile areas of western Liaoning Province and pollution of the environment, as well as the scarcity of soil sources for the reclamation of abandoned mine sites, such as open pits, the concept of large-scale agricultural resource utilization of iron tailings using iron tailings and other wastes to fill the mining pits and to construct a reclaimed farmland soil profile has been put forward. Iron tailings and normal farmland soils in the region were collected to analyze the basic physico-chemical properties, elemental content, and utilization feasibility of iron tailings through experiments. Based on the theories of “the niche of soil layer” and “the critical layer of the soil”, the mechanism of the function of iron tailings as soil profile material was explored. Combined with the existing practical experience, the technical model of a highly arable farmland soil profile with the structure of “loamy-sandy-clayey” was constructed, which is suitable for the semi-arid climate characteristics of western Liaoning Province. The results showed that: 1) The sand grain content of iron tailings is 28.49 % to 33.23%, the silt content is 66.11% to 71.64%, and the clay content is less than 1.0%. The pH of the iron tailings is weakly alkaline, the conductivity could meet the requirements of crop growth, and the heavy metal content is much lower than the Soil environmental quality Risk control standard for soil contamination of agricultural land (GB 15618 -2018), and it was rich in the beneficial trace elements required by crops, so it can be used as a material for reconstructing the soil profile. 2) The mechanism of iron tailings as soil profile construction material is to utilize its sandy structure with large intergranular pores for water storage to construct a soil moisture retention layer, forming a soil profile configuration system from top to bottom of "loamy (topsoil layer)-sandy (moisture retention layer)-clay (water barrier layer)", which plays the role of storing water and preserving moisture and provides water for the crop growth in the semi-arid areas. Meanwhile, it can be mixed in 25% amounts into the clayey topsoil to increase the thickness of the tillage layer while improving the soil texture. 3) The specific construction technology is to make full use of the waste rock, iron tailings, subsoil red clay, and stripped topsoil produced by iron ore mining, etc., and lay “large rocks with a grain size of 20～50cm (filled up to 1m from the surface) - crushed rocks with a grain size of 4～6cm (20～30cm) - red clay (20～30cm) - iron tailings (20～30cm) - stripped topsoil (20～30cm, mixed with iron tailings of about 25%)” in order from bottom to top, forming a ‘loamy-sandy-clayey’ soil profile configuration. By analyzing four reclamation cases in Jianping County, Liaoning Province, it is proved that the technology not only solves the ecological problems caused by solid waste accumulation in mining areas, but also increases the area of cultivated land, improves the quality of farmland, increases the yield of maize by 705 kg/hm², alleviates the contradiction between mining and agricultural land, and guarantees national food security. The new mode of ecological restoration in mining areas formed by the study can be popularized and applied in other similar iron ore mining areas according to the local conditions.