Long-term Rice Straw Biochar Alters Soil Pore Connectivity and Structural Anisotropy in an Acrisol

Purpose: Soil anisotropy, the variation in soil properties depending on measurement direction, affects water and air movement in the soil. Biochar is recommended to improve the properties of degraded tropical soils, but its long-term effects on the anisotropy of soil hydro-physical properties remain uncertain. Methods: Eight years after rice straw biochar amendment, we investigated its effect on the soil’s physical and hydraulic properties over time and anisotropy. The experimental layout was a randomized complete block design with four replicates. Biochar was applied only once in 2015 at rates of 0 and 30 Mg ha⁻¹, designated as B0 and B30, respectively. Intact 100 cm³ soil cores were sampled at 0–15 cm depth in vertical and horizontal directions to measure soil water retention, air permeability (k_a), relative gas diffusivity, pore tortuosity, and effective pore diameter. Results: Results indicate no significant (p > 0.05) effect of biochar on these properties relative to the control. However, in the vertical direction, B30 significantly (p < 0.05) reduced the degree of saturation at high moisture content. Findings suggest that rice straw biochar exerts a limited effect on k_a at low matric potential. Particularly, biochar amendment significantly influenced vertical pore organization (p = 0.03) and horizontal k_a (p = 0.03) at − 30 hPa matric potential, indicating biochar’s impact on pore connectivity and orientation. Conclusions: This study highlights the complex and variable effects of biochar on soil properties, emphasizing the need for a detailed examination of soil structural anisotropy and hydro-physical behaviour when assessing the long-term impact of soil amendments on soil physical health.