Spatial heterogeneity of biochar (segregation) in biochar-amended media: An overlooked phenomenon, and its impact on saturated hydraulic conductivity

While the use of biochar as a soil amendment is gaining popularity for environmental and agricultural purposes, spatial heterogeneity of biochar (segregation) in biochar-amended media and its underlying causes have been overlooked. In this study, for the first time particle segregation in biochar-amended media and its impact on the media's saturated hydraulic conductivity (Ksat) were investigated. Two uniformly graded media were amended with different sizes of a wood-based biochar under dry and wet conditions. While the intended biochar volume fraction (bf) was 17.5%, in dry-packed columns biochar was often segregated and the measured bf ranged from 7.5 +/- 0.8 SE% (SE = standard error) to 23.6 +/- 1.8 SE% across all spatial locations. If, however, 20% water (volume of water/bulk volume of packed media) was added to the mixtures during mixing, homogeneous packings were achieved. In dry-packing, segregation was governed by the difference in the physical properties of the media and the biochar: particle size, density, and shape. In wet-packing, segregation was prevented due to the inter-particle adhesion forces associated with water. Although X-ray computed tomography images showed that the presence of segregation altered particle distributions and pore morphologies, the Ksat for wet-packed and dry-packed columns were statistically identical. The results of this study suggest that laboratory methods for packing biochar-amended media should include moisturizing the mixture to inhibit particle segregation. Mixing under wet conditions is recommended for any type of soil and biochar and for any scale of application, in both the laboratory and field.

Automated summary

The study investigated particle segregation in biochar-amended media and its impact on saturated hydraulic conductivity. Dry-packed columns showed segregation due to differences in physical properties of media and biochar, while wet-packed columns prevented segregation due to inter-particle adhesion forces associated with water. Although segregation altered particle distributions and pore morphologies, the saturated hydraulic conductivity for wet-packed and dry-packed columns remained statistically identical. The study recommends mixing under wet conditions to inhibit particle segregation for effective applications in both laboratory and field settings.