Biowaste to boost urban agriculture

A project from the Institute of Agricultural Sciences (ICA-CSIC) analyzes the productivity and sustainability of rooftop agriculture in Madrid to contribute to the development of the circular economy. To this end, the institute's roof has been converted into a "green roof" for years, using substrates based on compost and other biodegradable organic waste, such as food by-products and algae, to grow vegetables. The results, published in Urban Forestry & Urban Greening, show an increase in the production of rooftop vegetables and a quality and nutritional profile similar to those obtained from field crops.

For three years, within the framework of the MadreenRoof project, a cultivation experiment has been carried out at the ICA-CSIC, located on the central Serrano Street in Madrid. Its objective was to evaluate the ecosystem services that an "intensive green roof" can provide, such as vegetable production, carbon sequestration, and the retention of nutrients and pollutants, among others. To do so, the research team "vegetated" the institute's rooftop using byproducts that underwent a composting process to create substrates and be able to grow crops in an inhospitable and underutilized space. "After more than four years, we have sufficient data to assess the potential of rooftop crops from an agronomic perspective," says Marco Panettieri, the ICA-CSIC researcher leading the study.

This result is the result of the development of growing substrates made from biowaste: invasive algae from the Atlantic coasts and food waste, such as coffee grounds and husks, composted with pruning remains and biochar (a carbon-rich byproduct obtained from the pyrolysis of biomass). "These substrates are presented as an ecological and sustainable alternative to traditional peat-based substrates (an organic material resulting from the partial decomposition of plant remains in humid environments), the use of which has a high environmental impact," explains Giuseppe Picca, a postdoctoral researcher at the University of Bologna (Italy) who developed his doctoral thesis at the ICA-CSIC.

This growing material was used to evaluate the production of lettuce and chard in the fall, and the Moruno de Aranjuez tomato variety in the summer, in collaboration with the Madrid Institute for Rural, Agricultural, and Food Research and Development (IMIDRA). The yield and quality of the vegetables produced in compost-based substrates were compared with traditional soil crops and with crops grown in commercial peat-based substrates. The results showed not only an increase of up to 70% in the production of rooftop tomatoes compared to those planted in the field, but also that their quality was comparable to that of tomatoes produced in field conditions. "This significant percentage increase in the production of the Moruno de Aranjuez tomato, native to the Community of Madrid, varied depending on the different compost mixtures and the year of cultivation," the researchers explain.

A new study, currently under review in the journal Ecotoxicology and Environmental Safety, indicates that the vegetables produced are safe and suitable for human consumption. This second study, the result of a collaboration between the Institute of Natural Resources and Agrobiology of Seville (IRNAS-CSIC), the Autonomous University of Madrid, the Université Gustave Eiffel in Nantes (France), and the ICA-CSIC, has monitored the content of heavy metals and metalloids in growing media, vegetable biomass, and leaching water for three years to assess the potential risks posed by the content in the media, their mobility and phytoavailability, and the effect of atmospheric deposition of particulate pollution.

"The addition of biochar significantly improved the structure and durability of the substrate, increasing its efficiency in water and nutrient retention," adds Laura Lozano de Sosa Miralles, a researcher at IRNAS-CSIC who leads this second study. Although a slight increase in trace elements due to atmospheric deposition was detected, the levels observed are well below legal limits, and simply washing the vegetables would further reduce their content, the researchers add.

These findings support the use of substrates developed at the ICA as an effective strategy for optimizing rooftop agriculture, enabling the use of underutilized urban spaces to sustainably produce food. Furthermore, this study represents a major step forward in promoting the circular economy and more efficient waste management, and offers a promising perspective for the integration of sustainable technologies into urban agriculture, contributing to the development of greener and more self-sufficient cities.

"Currently, within the framework of the MadreenRoof project, other ecosystem services are being evaluated, such as the impact on urban biodiversity, the insulation capacity of roofs, carbon sequestration, and flood mitigation thanks to the water retention capacity of rooftop crops," the researchers conclude.