Biochar effect on soil physical properties: A review

Biochar is charred biomass produced under limited or no oxygen (O₂) supply and at high temperature. Rich in aromatic carbon (C), biochar can persist in soil for hundreds or thousands years. Biochar application in agricultural soils has attracted an intensive global research with extensive reviews of biochar characteristics, effects on soil chemical, biological and physical properties and on crop growth. However, the work on the effect of biochar on soil physical properties is not as detailed as those on soil chemical and biological properties. In this study, relevant publications on the effect of biochar on soil physical properties in recent decades were searched and summarized. The results indicated that biochar application reduced soil bulk density, improved soil aggregate stability, field water holding capacity, soil available water content, and decreased saturated hydraulic conductivity through direct and indirect ways. Biochar has huge specific surface area and porosity, which directly influences soil physical properties such as reducing soil bulk density and increasing water holding capacity and soil available water content. Furthermore, biochar particles combine with soil minerals to change soil structure and create the proper environment for soil microbial and plant root growth, which ultimately affect soil physical properties. Indirectly, the effect of biochar on soil physical properties includes improved soil aggregate stability and saturated hydraulic conductivity. The effect of biochar on soil physical properties is related to biochar application rate, feedstock type, pyrolysis temperature, particle size, soil texture and duration in the soil. Generally, the higher the amount of biochar applied, the more significant are the induced changes in soil physical properties (bulk density, water holding capacity and saturated hydraulic conductivity). Woody biochar significantly increases saturated hydraulic conductivity, while grass biochar has no effect on saturated hydraulic conductivity. In addition, as the duration after biochar application increases, its effect on soil physical properties decreases. However, very limited studies have investigated this relationship under long-term filed conditions, and field experiments have been even relatively scarce. Consequently, the further studies should focus on the long-term effects of biochar application on soil physical properties in filed condition, and investigate the mechanisms behind the biochar effect on soil physical properties, which is essential to provide scientific evidence for the real agricultural production and ecological improvement. Furthermore, the relationships between biochar characteristics and soil physical properties are not fully known. Thus, there are needs to further investigate the effect of biochar application on the interaction of soil physical and chemical, biological properties and the mechanisms.