Effects of biochar addition on the hydraulic properties of compacted soils in mining areas

Abstract: Severe soil compaction often occurs with heavy traffic during mining, particularly for a large number of large-scale opencast mines in the Loess Plateau. An important indicator, the soil hydraulic property can be widely used to measure the soil pore conditions of compacted soils. Biochar can also be added to improve the land quality for higher crop yields in farmland in recent years. The environmental soil amendment has presented higher ecological and economic benefits, due to the specific properties from the special porous structure of biochar, including the large surface area, high porosity, low bulk density, and high organic carbon contents. As such, the soil bulk density can be reduced to increase the water holding capacity in the coarse texture soil. However, only a few studies focused on the biochar addition in the soil improvement in the mining areas, especially the hydraulic characteristics of compacted soil. The purpose of this study is to evaluate the impact of addition rates and particle sizes of biochar on the soil hydraulic conductivity under various compaction conditions in a mining dump. The soil sample was collected from Datong City in Shanxi Province of China. The experiment was then carried out under the four particle sizes of biochar (1-2, 0.25-1, 0.10-0.25, and <0.10 mm) with four addition rates (0, 4, 8, and 16 g/kg) and five compaction conditions (1.3, 1.4, 1.5, 1.6, and 1.7 g/cm3). A modified van Genuchten (VG) model was also established to determine the characteristic curve of soil water. The results show that all the correlation coefficients were above 0.960, and the standard deviations were below 0.015, indicating this model suitable for the characteristic curve of soil water after the biochar addition. Most macropores and mesopores with larger pore sizes were distributed to significantly improve the water holding capacity in the field. The specific values increased from the lowest 22.74% (1.3 g/cm3, 1-2 mm, 0 g/kg) to 37.56% (1.4 /cm3, 0.25-1 mm, 16 g/kg) with the amount of biochar addition. Furthermore, the initial soil water content increased from 0.135 cm3/cm3 (1.3 g/cm3, 1-2 mm, 0 g/kg) to 0.166 cm3/cm3 (1.3 g/cm3, 1-2 mm, 8 g/kg), where the maximum increase was 0.031 cm3/cm3 in an initial soil water content. Therefore, the biochar addition greatly contributed to the water holding capacity of compacted soil with less air entry for the better use of soil water. Correspondingly, the higher capacity of water holding was obtained at the lower bulk densities (1.3 and 1.4 g/cm3), and the larger particle sizes (0.25-2 mm). A combination of parameters was achieved for the optimal water holding at the high bulk density (1.5, 1.6, and 1.7 g/cm3), lower particle sizes (<0.25 mm), and higher addition (8, 16 g/kg). Consequently, the targeted application of biochar can be widely expected to effectively improve the water holding and water retention capacity of the soils with various compaction in the mining areas. This finding can also provide a promising basis for recovery the low-quality soil.