Biochar–Compost blends modulate trace element and nutrient dynamics in rooftop farming systems under Mediterranean conditions

The rising interest in Rooftop Agriculture (RA) has stemmed a demand for sustainable, lightweight alternatives to peat as plant growing media. Co-composting organic waste with biochar could represent a solution with reduced environmental impact. However, knowledge gaps remain regarding the food safety and environmental performance of these materials. This study examined trace element and nutrient dynamics in six substrates derived from three feedstocks: spent coffee grounds, coffee silverskin, and seaweeds, composted with and without biochar. Over three years, tomato (Solanum lycopersicum L., cv. Moruno de Aranjuez) and a mix of lettuce (Lactuca sativa L., cv. Romana) and Swiss chard (Beta vulgaris var. Cicla) were cultivated on a rooftop in central Madrid (Spain). To unveil the bigger picture behind element uptake by plants and leaching into drainage water, specific indices were calculated by grouping elements by risk-based categories. Results showed feedstock-dependent trace element and nutrient dynamics, with biochar reducing their plant uptake and leaching. Despite seaweed-based compost showing the highest arsenic levels, biochar lowered plant uptake by up to 40 %. Cadmium and lead in edible parts varied by year and substrate, but they remained within EU safety limits. Atmospheric deposition minimally affected lettuce trace element content, while washing reduced hazardous elements. Biochar improved nutrient retention, reducing phosphorus and nitrogen losses by 40 % and 25 %, respectively, over three years. These findings underline the potential of biochar-amended composts as sustainable, safe peat alternatives for RA, supporting crop production while mitigating environmental and health risks.