The Use of Biochar and Biosolids As Soil Amendments in a Greenroof System.

Greenroofs are constructed gardens on top of structures that help to improve energy use efficiency and increase greenspace in urban areas. Lightweight aggregrates and non-natural soils must be used on roofs due to structural loading limitations. These soils have a limited ability to retain water and nutrients. Some studies have shown improvements may be attainable with amendments like biochar (BC), a charcoal-like substance that has been created through the process of pyrolysis. Biosolids (BS) from municipal wastewater treatment were used as the feedstock for producing the biochar. This beneficial reuse helps to divert them from landfills while potentially improving greenroof substrates. For this experiment greenroof mesocosms were constructed and each fitted with a greenroof drainage system. They were then filled with clean greenroof substrate (control) and three amendments, biosolids at 5%, BC at 5% and BC at 20% by volume. Leachate pH, electrical conductivity, nitrate and soluble reactive phosphorus (SRP) were measured throughout the 85 day experiment. Sunflowers (Helianthus annuus) planted in the mesocosms grew at significantly (P=0.0495) greater rates with the BC-5% treatment compared to the control and BC-20%. In addition, leachate volume, nitrate and SRP was significantly lower with BC-5% compared to BC-20% and BS-5%. BC-20% resulted in the greatest volume of water leached through the system, counter to what was expected. An inverse relationship was found between the concentration of nutrients (N, P) in the leachate and total plant biomass, while substrate organic matter and magnesium concentration showed a positive relationship with plant biomass. These findings suggest that the BC added at 5% improves greenroof substrate nutrient availability, plant growth and also limits nutrient losses in leachate. Future research and practice should consider BC as a component of substrates to improve growth of plants and limit nutrient losses in greenroof systems.