PurposeThe objective of this study was to investigate the influence of apple tree biochar on the soil water retention curve (SWRC) of compacted loess, which is increasingly used in man-made earth structures in loess regions. Additionally, we aimed to elucidate the underlying mechanisms from both physicochemical and microstructural perspectives and propose potential directions for future research.Materials and methodCompacted biochar-amended loess (BAL) specimens with different biochar contents, dry densities or molding water contents were prepared, and their SWRCs were determined. Besides, the hydrophilicity, minerals, functional groups, structure and pore-size distribution (PSD) of loess, biochar and BALs were characterized for exploring the mechanisms by which biochar modifies the SWRC of loess.Results and discussionThe addition of biochar significantly improves the water retention capacity of compacted loess. With the increase of biochar content, both the saturated water content and air-entry value (AEV) increase, and the desaturation rate slightly increases. The influence of molding water content and dry density on the SWRC of BAL is similar to that on the SWRC of compacted loess.ConclusionsOn the one hand, the apple tree biochar studied is highly hydrophilic due to the presence of abundant oxygen-containing functional groups and negative charges on its surfaces, thus significantly enhancing the soil wettability. On the other hand, the biochar addition increases the volume of inter-aggregate pores and changes the type and size of aggregates, aggregates with a wide range of sizes are arranged more closely in BAL. Therefore, the soil water retention capacity and drainage capacity are improved. This study provides a theoretical basis for the applications of biochar in geotechnical or geo-environmental engineering in loess regions, however, further investigations are imperative.