Abstract: Biochar is increasingly used in soil improvement, pollutant remediation and other fields for its porosity, high specific surface area and strong adsorption properties. The addition of biochar can alter the pore structure of the soil, and subsequently change the water retention behaviour. Among many influencing factors, the biochar content and particle size have a great influence on the improvement effect. In order to study the effect of biochar content and particle size on water retention behavior of biochar-clay mixture, the soil-water characteristic curve in the wetting process was obtained for biochar-clay mixture with different biochar contents (0%, 5%, 10% and 15%) and particle sizes (>74, 40-74, 20-40 μm and <74 μm) at high suction range (3.29-286.7 MPa) by using the vapour equilibrium technique for controlled suction. The micro-pore structures were analyzed by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests. The results show that: 1) When the particle size of biochar is small, the water retention capacity of biochar-clay mixture improved obviously with the increase of biochar content; With the increase of biochar particle size, the water retention capacity of mixture is slightly affected by the amount of biochar. 2) When the content of biochar is small, the soil-water characteristic curves of biochar-clay mixtures with different particle sizes are basically the same, with the increase of biochar content, the influence of biochar with small particle size on the water retention capacity of biochar-clay mixtures has appeared out gradually. 3) The evolution of microscopic pore structure of biochar-clay mixtures further explains the influence mechanism of biochar content and particle size on its water retention capacity.

Key words: biochar, microstructure, particle size, content, vapor equilibrium technique, soil-water characteristic curve