Reduction of runoff through inclusion of BSI PAS100 compost and biochar substrate

The German green roofing guidelines, the FLL, currently recommend less than 12% of an intensive green roof substrate should constitute organic content. In partnership with WRAP, the RHS investigated the effects to plant growth and runoff volume and quality when substantially higher volumes of compost are included in a green roof substrate in combination with biochar.

Biochar is a biomass derived black carbon (also known as charcoal or wood-char). It is a fine-grained porous material high in organic carbon and largely resistant to decomposition in most environments. It can be used as a soil conditioner and also to enhance plant growth and retain nutrients in the soil by improving the soil’s physical and biological properties.

Evidence suggests that biochar (biomass derived black carbon) can offer nutrient and water retention benefits in soil (Lehmann et al. 2009), and may be beneficial in preventing loss of nutrients from green roof substrates. The RHS trial assessed the benefits in conjunction with PAS 100 compost.

Results suggested that the PAS100 compost performed favourably in reducing the volume of runoff, while concentrations of leachate initially appeared high but fell rapidly with each sampling. There was some evidence to suggest short-term increases in potassium and phosphorus concentrations in the leachate; most likely due to these nutrients being present in the biochar.

Overall, the trials suggest that the use of BSI PAS 100 food derived compost at inclusion rates of 35% and 50% in extensive green roof substrate mixes has potential, and it is possible that compost derived only from garden waste, which is generally expected to have a lower nutrient content, may be better suited to this type of extensive green roof scenario. The effects of the biochar were less clear and additional work into its use in green roof substrates is recommended.