Long-term effects of biochar on the hydraulic properties of soil: A meta-analysis based on 1–10 years field experiments

• The long-term effects of biochar on soil hydraulic properties were analyzed. • The effect of biochar on AWC increased in the second year and then decreased. • Woody and high-temperature biochar were more stable in improving soil AWC. • Biochar had positive effects on the K S of fine-textured soil in the field. • The recommended application rate of woody biochar was 20–40 t/ha. The improvement of soil hydraulic properties by the addition of biochar has been assessed by many researchers. However, the long-term effects of biochar on soil hydraulic properties under field conditions are still unclear. In this study, we conducted a statistical meta -analysis of 1–10 years of field experiment data from 61 studies published between 2009 and 2023. We sought to elucidate the long-term effects of biochar properties and application rates on soil hydraulic characteristics. Furthermore, we investigated the effects of applying biochar of different particle sizes to soils with different textures on soil hydraulic properties. According to the results, applying biochar under field experimental conditions significantly increased soil total porosity, saturated hydraulic conductivity, field capacity, and available water capacity, and obviously decreased soil bulk density. The effect of biochar on soil bulk density decreased as the duration of the field experiments increased. The effect sizes of biochar addition on soil total porosity, field capacity, and available water capacity reached maximum values in the second year and then gradually decreased in the following years because of biochar aging; however, a positive effect remained even after 3 years compared with the control. The key factors driving the long-term effects of biochar on soil hydraulic properties were biochar feedstock, application rate, and soil texture. Compared with crop residue biochar, woody biochar had stronger and more long-lasting effects on the hydraulic properties of soil. Compared with those of coarse-textured and medium-textured soils, saturated hydraulic conductivity, field capacity, and available water capacity of fine-textured soils were greater with biochar addition. This study recommends using woody biochar at an application rate of 20-40 Mg/ha to ensure its effectiveness in improving soil quality.