Changes in microstructural behaviour and hydraulic functions of biochar amended soils

Abstract

Biochar is increasingly used on agricultural soils to enhance productivity and to sequester carbon. However, there is limited research that directly quantifies the underlying mechanisms, particularly those assessing structural changes in amended soils. Also, there is scanty information on the long-term effect of biochar treatment on hydraulic response of soils. Our objective in this study therefore is to determine the effect of biochar treatment on two different soil types and to evaluate the influence on hydrological and mechanical properties that drive aggregate formation and stability. Test substrates were prepared by adding 2, 5 and 10% (by dry mass) of high temperature pyrolysed (HTP) biochar to fine-sand or sandy loamy silt soil material. The repacked cores were exposed to four cycles of wetting and drying in an experiment spanning about 300 days. Changes in the saturated hydraulic conductivity, aggregate sorptivity swelling and shrinkage behaviour, and microstructural stability during desiccation were also investigated. The results indicated that biochar amendment alters the pore structure with the saturated hydraulic conductivity being significantly increased (p<0.05) in the sandy loamy silt. Repeated wetting and drying significantly increased (p< 0.05) the repellency index of the amended substrates. Moreover micro structural stability was enhanced as the amount of biochar was increased and at lower matric potential. The partial loss of inter-particle cementation at a given pore water pressure due to increasing biochar amount was compensated by the noticeable addition of organic carbon. The combined effects of pore re-arrangement, enhanced particles surface area and improved microstructural stability in the amended soils produces better soil–plant–water environment. These findings proof that the effects of biochar amendment on pore structure, aggregation and stabilization will depend on the amount of biochar, the texture of the original soil material and the number of wetting and drying (WD) cycles.