From Municipal Green Waste to Agriculture: Biostimulant Production by Solid‐State Fermentation and Its Role in Improving Germination and Seedling Growth of Lettuce (Lactuca sativa)

Circular economy aims to transform waste into resources by closing organic cycles and reducing dependence on nonrenewable inputs. In this study, the use of a solid organic amendment containing indole-3-acetic acid (IAA) produced by solid-state fermentation (SSF) of municipal green waste composed of grass clippings and wood chips from parks and gardens and inoculated with Trichoderma harzianum was evaluated as a biostimulant for the improvement of germination and growth of lettuce seedlings (Lactuca sativa). Two organic amendments from different SSF processes were tested: the first one was obtained using tryptophan as a precursor for IAA production by Trichoderma harzianum and contained 119.02 μg IAA g ⁻¹ dw (FS-H), whereas the second one did not include tryptophan in the SSF process and resulted in a concentration of 11.80 μg IAA g ⁻¹ dw (FS-L). A control experiment without the addition of fermented solid was also performed. At 7, 14, 21, and 35 days after sowing (DAS), the application of FS-H, FS-L significantly improved germination indicators and biomass accumulation (shoot and root weights) compared to control. Furthermore, the content of photosynthetic pigments, including Chlorophyll a, Chlorophyll b, and carotenoids, was significantly higher in the treatments with high IAA content, with increases of 214%, 200%, and 175%, respectively, compared to those of control. Therefore, the use of these organic amendments acting as biostimulants favors seed germination, phenotypic changes, and chlorophyll content, while enhancing the yield of lettuce seedlings. This demonstrates the potential of the application of these organic amendments produced by SSF from organic waste inoculated with biostimulant producer strains, and it offers a sustainable and circular approach to improve agricultural yields.