Soil water retention, air flow and pore structure characteristics after corn cob biochar application to a tropical sandy loam

Soil structure is a key soil physical property that affects soil water balance, gas transport, plant growth and development, and ultimately plant yield. Biochar has received global recognition as a soil amendment with the potential to ameliorate the structure of degraded soils. We investigated how corn cob biochar contributed to changes in soil water retention, air flow by convection and diffusion, and derived soil structure indices in a tropical sandy loam. Intact soil cores were taken from a field experiment that had plots without biochar (CT), and plots each with 10tha⁻¹ (BC-10), 20tha⁻¹ without or with phosphate fertilizer (BC-20 and BC-20+P respectively). Soil water retention was measured within a pF range of 1 to 6.8. Gas transport parameters (air permeability, ka, and relative gas diffusivity, Dp/D0) were measured between pF 1.5 and 3.0. Application of 20tha⁻¹ led to significant increase in soil water retention compared to the CT and BC-10 as a result of increased microporosity (pores <3μm) whereas for soil specific surface area, biochar had minimal impact. No significant influence of biochar was observed for ka and Dp/D0 for the BC treatments compared to the CT despite the larger values for the two properties in the 20tha⁻¹ treatments. Although not significant, the diffusion percolation threshold reduced by 34% and 18% in the BC-20 and BC-20+P treatments, respectively, compared to the CT. Similarly, biochar application reduced the convection percolation threshold by 15 to 85% in the BC-amended soils. The moderate impact of corn cob biochar on soil water retention, and minimal improvements in convective and diffusive gas transport provides an avenue for an environmentally friendly disposal of crop residues, particularly for corn cobs, and structural improvement in tropical sandy loams.