Microbial inoculation enhanced the thermophilic phase and improved final maturity-related properties of coffee husk composting for tomato seedling substrate production.

Coffee husk composting is constrained by high lignocellulose content, slow decomposition, and residual phytotoxicity. Whether microbial inoculation can accelerate this process while producing a horticultural growing medium remains unclear. We compared coffee husks composted with a commercial microbial agent (a consortium of photosynthetic bacteria, lactic acid bacteria, yeast, filamentous fungi, and actinomycetes) (CHM) versus without (CH) over 180 days. The CHM treatment reached a higher peak temperature (55.6 °C vs. 43.5 °C) and at the end of composting achieved a numerically greater seed germination index (93.2% vs. 71.2%), indicating enhanced maturation and reduced phytotoxicity. High throughput sequencing showed that CHM did not drastically alter overall microbial α diversity but the final compost exhibited a marked reduction in the relative abundance of the genus Fusarium compared to CH. Beneficial bacterial genera such as Pseudomonas, Sphingomonas, and Alcaligenes showed a non-significant but consistent trend toward enrichment in CHM. When the mature CHM compost was blended with vermiculite, perlite, and decomposed cow manure, blends with lower compost inclusion (1:5 or 1:3 volume ratio of CHM compost to other components) supported tomato seedling growth comparable to a conventional peat based medium. These results demonstrate that microbial inoculation promote a more intense thermophilic phase and enhances final maturity in coffee husk composting, yielding a product with potential to partially substitute for peat in horticultural substrates. This approach offers a practical strategy for recycling coffee processing byproducts into a compost-based growing medium component.