Sustainability in urban agriculture: The role of biochar in enhancing the productive capacity of compost-based growing media for rooftop farming

Abstract

Rooftop agriculture offers ecosystem services and urban greening benefits, requiring lightweight, peat-free growing media. Composted organic waste, enhanced with biochar-rich materials, could serve as an effective component for these media. This study aimed to evaluate the agronomic performance of six different substrates made from organic wastes (spent coffee grounds, coffee silverskin, and seaweeds) composted with or without biochar as alternatives to peat for rooftop agriculture. Over three years, substrate fertility, crop yield, and quality were assessed using a tomato variety (Solanum lycopersicum L., cv. Moruno de Aranjuez) and a consociation of lettuce (Lactuca sativa L., cv. Romana) and chard (Beta vulgaris var. Cicla). A peat-based substrate and open-field conventional cultivation on soil served as controls. Co-composted substrates significantly increased productivity by 102 % compared to peat-based controls and by 43 % compared to open-field systems. For lettuce, compared to peat-based control, compost-based substrates without biochar yielded 380 % more in the first year, while those with biochar yielded 149 % more in the second year. Additionally, incorporating biochar during composting improved tomato quality, making it comparable to that achieved in open-field cultivation systems. In conclusion, co-composting organic waste with biochar to obtain growing substrates presents a sustainable alternative for city-focused rooftop agriculture, providing considerable horticultural yields that could reduce the environmental impact of the urban food needs.