Water Retention, Air Exchange and Pore Structure Characteristics after Three Years of Rice Straw Biochar Application to an Acrisol

Biochar has been suggested as soil amendment for improving soil structure and associated functions for agricultural production. We investigated the impact of rice straw biochar application on soil water retention (SWR), air movement through soil, and soil pore characteristics of a tropical sandy clay loam field. A field experiment was conducted at the University of Ghana's Forest and Horticultural Crops Research Centre, Kade, Ghana, which comprised three treatments: soil without biochar (B0), and soil amended with 15 and 30 Mg ha⁻¹ of biochar (B15 and B30, respectively). Three years after biochar application, we sampled intact 100 cm³ soil cores and measured SWR, air permeability (k_a) and gas diffusivity (D_p/D₀), and quantified pore characteristics: tortuosity (τ), effective pore diameter (d_B) and the number of air-filled pores in a given soil cross-section (n_B) at selected matric potentials. At all matric potentials (−10 to −15000 hPa), B30 considerably reduced SWR compared to B0, whereas the B15 had similar SWR as B0. Biochar did not significantly affect the plant available water (PAW). The B30 significantly increased k_a at −30 hPa relative to B15. At a given air-filled porosity, the B30 tended to have larger D_p/D₀ values compared to B0. Despite these improvements in soil air transport, the effect of the biochar treatment was marginal on soil τ, d_B and n_B. We suggest that, probably higher biochar application rates and longer time are needed to significantly improve PAW and soil pore structure characteristics, which control air and gas transport through the soil.