Three-dimensional fractal characteristics of soil pore structure and their relationships with hydraulic parameters in biochar-amended saline soil

•Biochar improved the soil pore structures and hydraulic characteristics of saline soil.•Biochar can be used as a soil amendment to improve coastal saline soil properties.•Multifractal parameters are useful tools for distinguishing the effects of pore structure on saline soil.•The relationship between soil pore structure and hydraulic behaviors is analysed. Due to poor soil quality, coastal saline soil limits soil productivity. Therefore, it is of great importance to find appropriate and efficient methods to improve the quality of coastal saline soil. Biochar has been widely used as a soil amendment to improve soil properties, but the effects of biochar amendment on soil pore structure and soil hydraulic properties are unclear. In recent years, with the development of fractal theory and computer technology, fractal geometry based on image analysis has been employed for quantitatively characterizing the structure and heterogeneity of soil pores. In this study, field experiments were established with three proportions of straw biochar (0, 2 and 5 %, w/w) to study the biochar amendment effects on soil structure and soil hydraulic properties. Three replicates were set for each treatment. The results showed that the addition of biochar to saline soil improved soil pore structures and soil hydraulic characteristics. The biochar amendment led to the strong pore spatial variation, complex pore structure and irregular pore shape. The biochar-amended saline soil had a high considerable water retention capacity (θS, AWC). Biochar had a long-term effect on saline soil improvement. For multifractal analysis, it is better to use the ΔD and Δα values to analyze the multifractal characteristics of soil. The heterogeneity of the soil pore distribution became stronger after biochar amendment. Multifractal parameters had significant changes with the time of biochar amendment. The changes of soil pore structure resulted in the improvement of soil hydraulic characteristics. The Ks value improved as the pore connectivity increased. The strong pore spatial variation, complex pore structure and irregular pore shape aided in increasing the θs value. The available water content value was improved as the number of soil pores, especially the micropores, increased. In response, this study proposes a new approach to analyzing soil pore structure to explain the association between the soil pore structure and soil hydraulic characteristics in biochar-amended saline soil.