Soil Hydraulic and Structural Responses to Biochar Amendment Under Deficit Irrigation in a Lowland Rice System

ABSTRACT

This study evaluated the effects of biochar amendment on the physical and chemical properties of an acidic sandy‐loam hydromorphic soil (humic Gleysol) under lowland rice cultivation in the Koussin‐Lélé area of Benin. A split‐plot experiment combined two irrigation regimes, continuous flooding (CF) and alternate wetting and drying (AWD), as main factors with biochar as the sub‐factor. Corn cob biochar (Bcc) and rice husk biochar (Brh) were applied at 5, 10, 15, and 20 t ha ⁻¹ , and soil responses were measured after two rice‐growing cycles (18 months after a single application). Biochar significantly improved soil hydraulic and structural properties. Soil water repellency declined markedly, with water drop penetration time decreasing from about 26 s in the control to ~15 s at 20 t ha ⁻¹ . Saturated hydraulic conductivity increased from 1.22 to 1.59 cm h ⁻¹ (+30%), with stronger improvements under AWD than CF. Bulk density decreased from 1.34–1.35 to 1.26 g cm ⁻³ , while total porosity increased from about 48% to 50%. Biochar amendment also enhanced soil water retention across the matric potential range, increasing water content at field capacity from 0.28 to 0.33–0.34 m ³  m ⁻³ and plant‐available water from 0.16 to 0.19 m ³  m ⁻³ (≈19%). Pore‐size analysis revealed that these improvements were primarily associated with the expansion of mesopores (≈2–30 μm). In parallel, aggregate stability increased by about 30%–35% at the highest biochar rates. The van Genuchten model accurately reproduced soil water retention ( r ²  = 0.98–1.00), confirming a consistent increase in available water capacity across irrigation regimes.