Alternative organic substrates enhance tomato growth in rooftop farming by promoting specific microbial communities

Background Urban agriculture represents an opportunity to alleviate some of the issues related with the increase in world population and the urbanization process associated. Productive rooftops offer a promising solution allowing to combine food production recycling organic waste and to provide green spaces without consuming urban soils. In addition, harnessing natural resources, including plant-associated microbiomes, is proposed as an effective approach to sustainably improve farm productivity and food quality. However, despite its importance for a better understanding of the interactions between substrates and plants, investigating microbes which inhabit different amendments (including compost and biochar) has received a little attention. Results Here, we explored the effects of feedstock materials (spent coffee grounds, coffee silverskin and seaweed biomass) and biochar-blending with compost on horticultural substrate properties, microbial communities, and tomato production. Peat was used as a conventional substrate, for comparison. Substrate properties were mainly defined by the feedstocks, but adding biochar during composting enhanced the quality of the end-products. Overall, alternative substrates differed from peat mainly by showing higher phosphorus content, pH and electric conductivity. Concerning microbial communities, biochar blending was less relevant than the feedstocks. Even though alternative substrates presented higher bacterial diversity than peat, their taxonomic composition remained similar among treatments, reflecting the functional redundancy of the bacterial communities. In addition, we identified specific communities associated with each feedstock, being those associated with composted seaweed biomass positively and significantly correlated with tomato yield. Conclusions The studied substrates influenced tomato growth and yield directly via their chemical and physical properties and by influencing microbial community composition. Thus, our results reveal the importance of using the appropriate organic substrate for enhancing the effectiveness of rooftop agriculture while increasing microbial diversity in urban unexploited areas.