Green roofs are an effective green infrastructure tool to reduce stormwater runoff in cities, but their considerable weight is a constraint on widespread uptake. Consequently, lightweight green roof substrates with high water retention are desirable. Biochar addition to green roof substrates may reduce green roof weight and improve stormwater retention by increasing water holding capacity. However, the effects of biochar on substrate properties can vary due to differences in biochar particle size and amendment rate that influence substrate porosity and therefore substrate water and air-content. Additionally, differences in the feedstock used to produce biochar also mean that biochars are highly variable in their internal porosity, affecting their water retention and weight. To determine how biochar particle size, amendment rate and feedstock density influence substrate water retention, bulk density, air-filled porosity and infiltration we tested substrate properties of a scoria-based green roof substrate mixed with two different biochars contrasting in feedstock type, at three different biochar particle size fractions (coarse 2 mm - 10 mm, fine <2 mm, mixed <2 mm - 10 mm) and four amendment rates (10, 20, 30, 40% v/v), and compared them with an unamended control. Biochar particle size and amendment rate influenced substrate properties more than feedstock type. Finer biochar particle size improved water retention but decreased infiltration and air-filled porosity, while larger biochar particle size moderately improved plant available water and maintained infiltration and air-filled porosity when compared with the control. Greater amendment rates reduced substrate weight regardless of particle size and feedstock type. While biochar with finer particle size can improve water retention, it also reduces air-filled porosity and infiltration, which may cause waterlogging and necessitate additional stormwater controls in locations with high rainfall intensities. Therefore, coarse biochar amendment is preferred as it improves plant available water, reduces substrate weight and ensures good drainage for plant performance. Further work is required to evaluate the effects of biochar amendment on plant growth and hydrological performance in full green roof system configurations.
