Biochar granulation and particle size influence hydrological performance of green roof substrates

Green roofs are increasingly implemented in cities to improve stormwater management and provide additional ecosystem services. Biochar, a carbon-rich material derived from pyrolyzed biomass, has emerged as a promising substrate additive to improve hydrological performance of green roofs. Biochar granulation and particle size modification have been suggested as effective approaches to mitigate erosion losses but research on the effects of biochar post-processing on hydrological performance remains scarce, and data on fundamental hydraulic properties of biochar-amended green roof substrates is entirely lacking. We investigated how biochar granulation and particle size influence surface and pore characteristics and hydraulic properties of green roof substrates in laboratory experiments. Granulated biochars exhibited collapsed surface morphology and pore structure, resulting in consistent hydrological responses across particle size ranges, likely due to the homogeneous structure formed during granulation. In contrast, unprocessed biochars showed a significant increase in Brunauer-Emmett-Teller (BET) specific surface area and pore volume with decreasing particle size. The amendment of granulated biochars improved substrate field capacity, suggesting potential enhancement of stormwater retention. The addition of fine (0.5–1 mm) and intermediate-sized (2–2.8 mm) unprocessed biochars also increased plant available water. Although reduced saturated hydraulic conductivity associated with fine unprocessed biochars may improve green roof detention capacity, fine biochar particle size may increase erosion losses and periodic waterlogging. We conclude that the application of either granulated biochar or intermediate-sized unprocessed biochar may provide a balanced enhancement of stormwater retention and detention capacities, as well as improved plant available water in green roof systems.