Microbiomes as Drivers of Soil and Plant Health

The rhizosphere microbiome plays a crucial role in ecosystem functioning by contributing to nutrient cycling, soil structure formation, and plant defence against pathogens and toxins. However, despite significant advances in sequencing technologies, much of its diversity and functional potential remains poorly understood. This thesis investigates how two circular economy solutions, spent coffee grounds (SCG) and compost derived from the co-composting of organic urban waste with compostable bioplastic packaging, affect soil microbial communities through both laboratory and field experiments. In the first study, an innovative biofertilizer produced from SCG waste was tested on Lactuca sativa (lettuce) plants under controlled growth chamber conditions. While raw SCG negatively affected plant growth, causing chlorosis and leaf damage, its transformation into a tailored biofertilizer led to healthier, more vigorous plants with higher concentrations of bioactive compounds. DNA metabarcoding of the lettuce rhizosphere revealed that the novel formulation enriched beneficial bacteria (e.g., Pseudomonas, Bacillus) and protist predators, while also increasing the availability of nitrogen, phosphorus, and potassium in the soil. The second study involved a field trial conducted over a single growing season, comparing wheat plants and their rhizosphere microbial communities in soils amended with either compost obtained from organic urban waste and compostable bioplastic packaging (BioP), standard compost without bioplastics (Comp), or unamended soil (Ctrl). Both compost treatments promoted the proliferation of plant-growth-promoting bacteria and decomposer fungi, indicating a positive impact on soil microbial communities. Notably, no adverse effects were observed from the use of BioP compost. Overall, these findings highlight the potential of reusing spent coffee grounds and compost derived from compostable bioplastics and organic waste as sustainable soil amendments. By enhancing beneficial plant–microbe interactions and improving soil health, these circular strategies offer promising pathways toward more sustainable agriculture