Effects of biochar on hydraulic parameters and shrinkage-swelling rate of silty clay

Abstract: Biochar is an effective soil amendment. Its effects on soil chemical properties have been widely studied but little is known about its influence on soil hydraulic properties. In this study, we aimed to investigate sizes and addition proportion of biochar on silty clay loam. The loam was collected from 0-10 cm paddy field in Qujialing research base of State Key Laboratory of Water Resources and Hydropower Engineering Science ( which located on Jingzhou City, Hubei province). Two kinds of biochar sizes were designed including fine biochar with size of 0.013-0.048 mm and coarse biochar with size of 0.3-0.7 mm. They were added to loam in two ratios of 3% and 6%, respectively. The hydraulic parameters and shrinkage-swelling rate of silty clay loam were determined. The centrifugal method was used to measure soil water characteristic curves, and vernier caliper was used to measure shrinkage properties in the process of the centrifugation. Results showed that: 1) Both fine and coarse biochar addition could decrease water retention ability of tested soil, but the effects of biochar sizes and addition ratios were not significantly different; Biochar addition increased the proportion of very fine (<0.3 μm) and middle-large (＞30 μm) pores in total pores, but increased that of minor and small pores(≥0.3-30 μm); Fine biochar addition could enhance development of pores ≥0.3-5 μm while coarse biochar addition facilitated pore development of ≥5-30 μm; In general, addition ratios did not significantly affect pore distribution; 2) Biochar application decreased the saturated moisture content and wilting coefficient of tested soil but increased effective moisture content in soil; Fine biochar had the highest capacity to increase soil effective moisture, and the effects were not affected by biochar addition ratios; Soil water holding capacity was greatly affected by coarse biochar addition but not by fine biochar addition, and coarse biochar could decrease water holding capacity; 3) Both fine and coarse biochar decreased the soil shrinkage rate during centrifugal process, and the effect of fine biochar on shrinkage rate of soil was more significant than that of coarse biochar; 4) Fine biochar had no effect on swelling rate of soil, and coarse biochar decreased swelling rate of soil; 5) Biochar application decreased saturated hydraulic conductivity of soil; 6) The treatments of 3% and 6% biochar addition rates both for fine biochar and coarse biochar didn't show significant difference in most of the parameters studied here. Therefore, 3% biochar addition rate was recommended for practical application. In conclusion, biochar addition shows positive and negative influences on soil. For example, it could decrease saturated hydraulic conductivity of soil, thereby affecting rice production. It is necessary to alleviate the adverse effects by other drainage measures in field.