LAUSR

Purpose Biochar amendment is considered as a soil management practice worldwide for the sustainable agricultural production by improving soil physical property, fertility, and microbial activity. However, the change of soil structure and hydraulic conductivity over time after applying biochar to expansive clayey soil under field conditions are unclear. Materials and methods In this study, biochar was applied to soil at the rates of 0% (CK), 1% (BC1), and 3% (BC3) (w/w) to assess the variation in soil physical properties after day 10, day 193, and day 375, respectively. Results and discussion The results showed that the total soil porosity estimated based on bulk density and the average particle density of 2.65 g cm −3 was significantly higher than the total porosity as measured by water retention curve ( P  < 0.05). Compared with day 10, the measured total porosity at days 193 and 375 significantly decreased by 5.09% and 7.38%, respectively, under CK; however, that significantly increased by 2.14%/2.60% and 1.85%/3.60%, respectively, under BC1/BC3 ( P  < 0.05). The average bulk density in 1 year was significantly lower 3.30%/6.99% under BC1/BC3 than CK ( P  < 0.05). Compared with CK, soil pores (5–30 μm) significantly increased by 12.71% under BC3 and soil pores (> 300 μm) significantly increased by 114.25%/164.58% under BC1/BC3 after 375 days ( P  < 0.05). There were no significant differences in the saturated hydraulic conductivity among all treatments after 1 year ( P  < 0.05). Conclusions Our findings indicate that the estimated total porosity is not suitable for evaluating the change of soil porosity over time after biochar application. Biochar in size less than 2 mm can improve soil structure in expansive clayey soil under field conditions. However, these positive changes may be insufficient to facilitate the improvement of saturated hydraulic conductivity after 1 year at the biochar application rates of 1% and 3%. Our findings provide insights into the application of biochar as a soil amendment for management practices in expansive clayey soil.