Influence of feedstocks and pyrolysis on biochar’s capacity to modify soil water retention characteristics

• An extra 1.4–2.6 days available for crop water use if biochar were to be applied.
• Biochar additions increase water content in the upper portion of the water retention curve.

The incorporation of biochar has an effect on soil water retention characteristics due to rearrangement of soil structure and geometry. But information is very limited on how feedstock and pyrolytic processes influence biochar modification of soil water characteristics. A laboratory study was conducted to test if (i) biochar improves water retention capacity in selected soils, and if (ii) different biochar production methods and feedstocks uniquely influence water holding capacities. Two contrasting soil series, Maddock (Mixed, frigid Entic Hapludolls, sandy loam) and Langhei (fine-loamy, mixed, superactive, frigid Typic Eutrudepts, clay loam) were used in this study. Biochar materials were produced from two biomass feedstocks, corn stover and switchgrass, using fast pyrolysis and slow pyrolysis. Biochar materials were incorporated into soils at 40 g kg−1 soil. A fit of the van Genuchten equation to the water retention curve showed a significant difference in the water retention curve corresponding to the region associated with macropores and mesopores after adding biochar to the Maddock soil. Several of the biochar treatments increased plant available water holding capacity on both soils. These included the fast pyrolysis corn stover and switchgrass biochars. Assuming a transpiration rate of 5 mm d−1 the results indicate an extra 1.4 to 2.6 days available for crop water use if biochar were to be applied at study rates to a 0.2 m depth. This would extend the interval between precipitation events needed for crop growth and survival on severely eroded soils of low water holding capacity. These results will need to be confirmed under field conditions before recommendations can be made for use in rehabilitating eroded soils.