Biochar as a Substrate Amendment in Green Roofs A Comparative Analysis of Nutrient Retention, Water Management and Application Techniques

Abstract

Incorporating green roofs offers sustainable solutions to various urban problems, but they also present challenges such as nutrient leaching, which can impact the environment and cause eutrophication. Biochar, a carbon-rich by-product of pyrolyzed biomass, emerges as a potential solution to this problem. This carbon-negative technology can enhance soil fertility, water retention, and mitigate nutrient loss. However, the properties and subsequent performance of biochar are heavily influenced by its feedstock and pyrolysis temperature, resulting in significant variability. This thesis compared the effectiveness of two types of biochar, conventional wood-chip biochar and granulated crop-based biochar, in improving the performance of green roofs. Initial investigations suggested superior adsorption properties in wood-chip biochar, owing to its high surface area and porosity. However, considering the application rate based on volume, revealed the granulated biochar to be more beneficial for adsorption. Granulated biochar, characterized by higher oxygen, nutrients, and ash content, demonstrated better chemical affinity which was pivotal for adsorption, outweighing surface area and porosity. No significant differences in water retention between the two biochars or the unamended substrate were observed, contradicting the expected behavior of biochar. The long-term effects of different biochars, considering aging and the impacts of microbial activity, were not analyzed in this study. For short term adsorption efficiency, the optimal technique was to apply biochar as a layer at the bottom of the substrate. Scientific literature, however, indicated that plants benefited the most when biochar could interact with the roots. Combining both options was thus suggested as a solution for maximizing plant benefits, nutrient retention, and carbon sequestration. The literature review suggested that more studies on biochar addition to green roofs in the 10-15% (v/v) range are needed, and both plant health and nutrient retention should be addressed. The thesis concluded with the proposition that better understanding of biochar could enhance green roofs, offering an environmentally beneficial solution to the issue of nutrient leaching.