Biochar-based organic substrates enhance tomato growth by promoting specific microbial communities in rooftop farming

Urban agriculture represents an opportunity to alleviate some of the issues related with the increased urbanization and global human population. Productive rooftops offer a promising solution that combines food production and recycling of organic waste, while providing 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, little attention has been given to the importance of selecting the appropriate organic substrate to enhance plant-microbe interactions and rooftop farming in urban areas. Here, we study the influence of different organic substrates on tomato, and its associated microbial community, in an open-air rooftop farming setup. Specifically, we evaluate combinations of peat with a high quantity (50% of volume) of compost derived from different feedstocks (seaweed biomass or coffee byproducts), and how biochar-blending alters these compositions.

We found that substrate properties were mainly defined by the compost feedstocks. Biochar blending had a minor influence on substrate composition, although it enhanced tomato yield. Overall, alternative substrates differed from peat mainly by showing higher phosphorus content, pH, and electric conductivity. Similarly, compost feedstock had a greater impact on microbial communities than biochar blending. Even though alternative substrates presented higher bacterial diversity than peat, the taxonomic composition was similar across alternative substrates, reflecting the functional redundancy of the bacterial communities. In addition, we identified specific microbes associated with each compost feedstock. The abundance of bacteria and fungi associated with composted seaweed biomass was positively associated with tomato yield. Among them, bacterial genera like Acidibacter, Altererythrobacter, Amaricoccus, Luteitalea, Microvirga, Pedomicrobium or Pseudorhodoplanes stood out presenting strong correlations with tomato yield.

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.