Impacts of Woody Biochar Particle Size on Porosity and Hydraulic Conductivity of Biochar-Soil Mixtures: An Incubation Study

Biochar improves soil performance by altering soil physical characteristics. To understand the effect of biochar particle size on soil hydrology, we measured the hydraulic conductivity (Ks) of biochar-soil mixtures. To better understand how physical properties influenced Ks, skeletal and (bulk density (, the water saturation (S), and the porosity (Ф) of mixtures were measured. A sandy loam soil was mixed with two woody biochar particle sizes (1–2 and <1 mm). All used biochars significantly decreased Ks compared to soil but this reduction for finer biochars was more distinct. When finer particles of low and high-tempered biochars were applied Ks decreased 14.3% and 18.9% while coarser particles reduced Ks 13.9% and 17.5%, respectively. Also, all biochars increased total, effective, inter and intraporosity of mixtures compared to control after 180 days. Negative correlation between Ks and porosity demonstrates the increase of porosity isn’t necessarily providing flow paths and thus doesn’t have a positive influence on Ks. Decrease of Ks through the addition of fine biochars was because they filled spaces between soil particles, which increased tortuosity and reduced pore throat size. Also, the decrease of Ks associated with coarser biochars was caused by the bimodal particle size distribution, resulting in more compact packing and tortuosity.