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Changes in mechanical and thermal properties of biodegradable mulches based on plasticized PLA/PHB filled with 3 wt% of carbon black were investigated during natural ageing in real soil environments after application for one season. The natural ageing process changes were evaluated under three different conditions of the mulching films: (a) buried in the soil, (b) exposed to sunlight, and (c) shadowed by plants. In a parallel study, the film was irradiated by a UV–Vis lamp under laboratory controlled test conditions at constant temperature and humidity. All samples were analyzed using GPC, DSC, DMTA and tensile tests. The plasticized PLA/PHB/CB film buried in soil lost its tensile properties faster than when shadowed by plants or exposed to sunlight. The deterioration of mechanical properties during degradation resulted in significant decreases elongation at break, while Young’s modulus values and tensile stress increased. Assessment of the mulching effect on the harvest of sweet pepper fruits, as well as carotenoids and vitamin C contents, was also performed. The obtained results showed that plasticized PLA/PHB/CB film has the potential for application for one season mulches.