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Biochar is a carbon-rich material produced mainly from plant- and animal-based biomass through thermochemical conversion. It is often amended to soil for methane oxidation in landfill cover soil and for improving crop growth and yield by enhancing soil fertility. The application of biochar for removing organic and inorganic pollutants from soil and water is also well established. Biochar could be of potential use as an engineering material for enhancing the stability and performance of bioengineered structures that mainly consist of soil and vegetation. Bioengineered structures are commonly adopted in many parts of the world due to their multiple beneficial features. The stability and performance of these structures depend on the soil engineering properties and vegetation performance. However, the performance of vegetation is interrelated with the soil engineering properties. Therefore, the effects of biochar on soil engineering properties and vegetation performance, i.e., the soil-biochar-plant interaction, need to be studied and summarized. In the present study, the effect of biochar on the soil engineering properties and vegetation performance was reviewed. The review of the literature revealed that the amendment of biochar affects the soil engineering properties and vegetation performance by altering mainly soil pore system and fertility. However, this depends on many factors, such as soil type, biochar type, pyrolysis temperature, particle size of biochar, biochar amendment rate, aging of biochar, and the type of vegetation grown. Furthermore, the reviewed or existing literatures are mostly for agricultural soil, which is different from the soil state in bioengineered structures in terms of compaction density, suction range, and design life or duration. In addition, the properties of biochar are highly variable with its production process. Hence, the factors of soil state in bioengineered structures and the optimized production process of biochar need to be considered for maximizing the long-term performance of bioengineered structures. [ABSTRACT FROM AUTHOR]

• A detailed review of soil water retention characteristics (SWRC) of biochar-amended soil is presented. • Biochar feedstock type, pyrolysis temperature, particle size and aging has a strong influence on the SWRC. • The SWRC of biochar-amended soil also depends on soil type and corresponding compaction condition. • Correlations among SWRC parameters and biochar properties were developed and presented. • Such a study will be quite handy while applying biochar-amended soil in geotechnical and agricultural applications. Biochar is a stable carbon-rich product obtained through pyrolysis of plant and animal-based biomass. Soil is often amended with biochar due to the beneficial features, such as carbon sequestration, improvement of crop growth and yield by enhancing soil fertility, removal of an organic and inorganic pollutant from the soil, and improvement of water retention properties of the soil. Fundamentally, the amendment of soil with biochar alters the soil water retention characteristics (SWRC) of the soil. This is primarily due to the inherent properties of biochar that includes large specific surface area (SSA) by the intra-particle pores, and the functional groups present in biochar surface. SWRC is vital for both agricultural soil and soil in geotechnical or geo-environmental engineering structures due to the presence of vegetation. The effect of biochar on the SWRC of agricultural soil was extensively studied. However, it is rarely studied for soil in geotechnical or geo-environmental engineering structures, such as landfill cover, bioengineered slope, embankment, green roof, etc. Soil in these structures subjects to a distinct condition, in terms of compaction state and design life compared to agricultural soil. Therefore, a comprehensive gathering of information on the effect of biochar on the SWRC and the factors controlling the biochar influence is needed for potential investigation and application of biochar amendment for soil in geotechnical or geo-environmental engineering structures. In this article, the effect of biochar amendment on the SWRC of soil has been reviewed and summarized by considering various factors controlling the influence of biochar on the SWRC. The literature review revealed that the amendment of soil with biochar could improve the SWRC while few occasions not influence the SWRC. Factors, such as feedstock type and pyrolysis temperature, particle size of biochar, soil type, and compaction state have strong bearing on the influence of biochar on the SWRC. [ABSTRACT FROM AUTHOR]