Biochar effects on soil hydraulic properties: high-resolution analysis for contrasting soil textures

Biochar amendment is widely promoted as a means to sequester carbon while improving soil physical properties. Its hydraulic effects depend strongly on particle size and application rate. Available studies mainly focus on enhanced water retention in sandy soils. Studies that include fine-textured soils and quantify effects on unsaturated hydraulic conductivity remain limited, hampering the development of reliable management strategies. Here, we present the effects of biochar addition on the soil hydraulic properties (SHP) of two agricultural topsoils with contrasting textures. Water retention and hydraulic conductivity of a loam and a loamy sand were measured after amendment with wood-derived biochar of three particle sizes (<0.5, <2, and <10 mm) applied at three dosages (1, 2 and 4 wt.%). All samples were packed under identical force and characterized over the full moisture range using the simplified evaporation method, complemented by saturated conductivity measurements and dew-point measurements of dry-range water retention. A comprehensive soil hydraulic model incorporating adsorption and film flow was fitted to all data, enabling systematic analysis of how biochar size and amount affect hydraulic behavior. Relative to the controls, all biochar treatments increased porosity and saturated water content. Saturated hydraulic conductivity increased by up to 200% for the loam but decreased for the sand. In the loam, biochar application improved air capacity by up to 6 vol.% but had no effect on plant-available water. In contrast, biochar quantity and particle size had no effect on the air capacity of the sand, but increased its available water content by up to 3 vol.%. Higher biochar application rates were strongly associated with lower air-entry values, reduced bulk density, and a broader pore-size distribution. This indicates a shift toward smaller pores in the loamy sand and larger pores in the loam. Smaller biochar particles slightly increased unsaturated hydraulic conductivity between 100 and 300 cm suction for both soils, but reduced water retention in the sand at suctions greater than 100 cm compared to coarser biochar. Overall, our findings demonstrate a substantial influence of biochar on soil hydraulic conductivity and water retention, with effects being stronger in coarse-textured soils and more sensitive to application rate than to particle size.