The influence of biochar particle size and concentration on bulk density and maximum water holding capacity of sandy vs sandy loam soil in a column experiment

Biochar application to agricultural soils has been proposed as a way to increase crop production by improving soil chemical and physical properties. Liming potential and improved nutrient exchange on biochar surfaces are the most reported mechanisms. Wherever crops experience drought stress, improvements in soil water holding capacity (WHC) might also be an important mechanism. However, reported effects on soil structure and WHC are mixed. Therefore, we studied the effects of biochar on soil bulk density (BD) and WHC in a laboratory column study using two agricultural soils from Portugal: a sandy and a sandy loam soil. Mixed woody feedstock was pyrolysed at 620 °C, creating a wettable biochar that was used unsorted as well as sieved into large (2–4 mm) and small (0.05–1.00 mm) particles, mixed into the soils at 1, 5, 10 and 20% (by volume), and incubated for 10 days at field capacity to allow aggregation. Soil samples were analysed for BD and WHC using soil columns. We found biochar to decrease soil BD and increase maximum WHC, expressed as gravity-drained equilibrium water content, for both soils. The sandy soil was more responsive with significant effects at the lowest application rate (1%), while the sandy loam soil started to show significant effects at 5% biochar. Small biochar particles reduced the BD of sandy soil more, while large biochar particles caused a greater reduction in the BD of the sandy loam soil. The effect of biochar particle size on WHC was less clear, except for small particles at 20% volumetric concentration, which showed a 60% increase in gravimetric WHC. When expressed as total soil water storage (SWS), 20% biochar incorporation to 15 cm depth would increase the total SWS of sandy soil from 0.56 mm (control) to 0.83–0.91 (mm), and of the sandy loam soil from 0.56 to 0.79–0.96 (mm), depending on biochar particle size. Our results suggest that biochar particle sizes can be used to achieve specific effects in soils, while mechanisms and trade-offs (agro-economic and environmental) need further exploration.