Gas permeability and water retention of a repacked silty sand amended with different particle sizes of peanut shell biochar

Abstract

Biochar has been used as an eco-friendly enhancer to alter soil microstructure and improve the mechanical and hydraulic properties of soil. Recent studies observed that variation in biochar particle size can affect its ability to alter soil microstructures. This poses a challenge to select the proper biochar particle size to manipulate soil properties to favor agricultural applications including crop cultivation or engineering design, such as landfill cover. This study experimentally investigated the influence of biochar particle size on soil microstructures and its effects on water retention and gas permeability (k_g) of a repacked biochar-amended soil (BAS). Peanut shell biochar with four different sizes, ranging from finer than 0.25 to >2 mm, was amended into silty sand. Change in microstructure of BAS was observed using a mercury intrusion porosimeter. Results indicated that the particle size of biochar played a significant role in altering soil microstructures. The addition of smaller biochar particles (i.e., <0.25 mm) decreased soil mesopores and macropores by ∼19%, resulting in greater water retention and reduction of k_g by ∼31% at ∼0.35 air-filled porosity. However, the mesopores and macropores of soil amended with larger biochar particles (i.e., >2 mm) increased threefold. This resulted in a reduction of water retention at low matric potential (i.e., lower than −3 kPa) and increased k_g by up to 93% at ∼0.35 air-filled porosity. These findings indicate that application of smaller biochar particles can enhance the water retention ability of the silty sand and reduce soil k_g, which is beneficial to the water and air management of the biochar-amended soil layers (i.e., as an aeration layer or sealing layer).