Increasing concerns about environmental issues have recently intensified the search for sustainable alternatives to conventional plastics that minimize ecological impacts. This study evaluates the biodegradability, compostability, and ecotoxicity of a PLA-based biocomposite containing 30–40% micronized cellulose fibers. The material complied with the European limits for fluorine and heavy metals. Biodegradability was assessed through a respirometric test under thermophilic conditions, achieving 81% degradation in 155 days. Thermophilic compostability was evaluated by monitoring the disintegration of injected products made from the biocomposite pellets and cut into pieces with thicknesses of 1.0 mm and 2.1 mm, revealing that increased specific surface area prolongs composting time. Ecotoxicity was tested through seed germination and plant growth assays on barley, onion, sunflower, tomato, and wheat using the biocomposite mature compost mixed (25% and 50%) with a TÜV Austria certified soil. Results showed species-dependent effects: sunflower germination was enhanced, while other plants experienced slight growth delays. No severe phytotoxicity was observed, except for barley and wheat. Despite the proven biodegradability and compostability, the biocomposite product’s dimensions influence disintegration and decomposition rates. Furthermore, compost applications may have variable effects on plant development. These findings improved knowledge about sustainable materials performance, raising awareness about more responsible design, consumption, and disposal strategies.

Poly(lactic acid) (PLA) is a promising bio-based polymer for melt-spun fibres, but its low melt strength, brittleness and narrow processing window limit broader use. This study evaluates PLA/cellulose biocomposites for monofilament melt spinning by comparing micronized bleached Eucalyptus kraft pulp-derived fibres (P200) with microcrystalline cellulose (MCC) as a particulate reference, within a fixed compatibilizer/processing-aid formulation. Composites containing different cellulose contents were prepared by melt compounding and assessed by thermal analysis, oscillatory rheology, mechanical testing and fracture morphology; selected 5 wt.% formulations were then processed into monofilaments. Cellulose incorporation increased the onset degradation temperature of the PLA reference, while the rheological and mechanical responses depended strongly on reinforcement morphology. P200 promoted stronger low-frequency melt structuring, stiffness and crystallization tendency, but also a greater reduction in ductility, with notched impact strength decreasing significantly only at the highest P200 loading. MCC produced a more moderate composite-level response. Stable continuous monofilaments were obtained from both selected cellulose-containing formulations, although neither matched the drawable window or draw-induced mechanical development of the PLA reference formulation. Among the cellulose-containing systems, the 5 wt.% MCC formulation retained the more favourable balance between reinforcement and melt-spinning processability.