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Compacting engineered soil media is necessary to ensure its stability in many urban contexts, such as in road embankments. However, compaction can reduce infiltration rates, increase runoff volumes, and impede vegetation growth. Amending soil media that is to be compacted by mixing it with biochar may largely mitigate the consequences of compaction, effectively helping road embankments and similar areas function as green stormwater infrastructure. Here, we report the results of a lab experiment intended to determine how biochar particle size and application rate influence the hydraulic properties of compacted roadside media. Specifically, we amended a loamy sand collected from the field with biochar ranging from 0 to 6% (w/w) and that was either unsieved or sieved to remove most coarse particles. The addition of biochar with many coarse particles (>2 mm) improved the saturated hydraulic conductivity (Ksat) of the medium postcompaction, but this effect diminished at biochar dosages above 3% (w/w). In contrast, the addition of biochar with few coarse particles did not improve Ksat postcompaction. These changes likely arose because unsieved biochar facilitated the addition of interconnected pores and did so to a greater extent than particle breakage led to pore restriction. Also, biochar addition improved water retention in the plant available suction range postcompaction, whether the biochar was sieved or unsieved. Amendment with relatively coarse biochar could therefore help compacted roadside soil media manage stormwater while also reducing plant water-stress in a frequently water-limited setting.
Caplan et al. (Fri,) studied this question.