Effects of Biodegradable Mulch Films with Different Thicknesses on the Quality of Watermelon Under Protected Cultivation

Biodegradable mulch films (BDMs) have emerged as a promising alternative to conventional polyethylene (PE) films in modern horticulture, yet the effect of film thickness on crop performance remains inadequately understood. In this study, a two-year field experiment (2023–2024) under protected cultivation was conducted to evaluate BDMs with thicknesses (0.006, 0.008, and 0.010 mm) for watermelon production in Beijing, China. The results showed that all BDMs enhanced soil temperature and moisture compared to bare soil (main effect of mulching, p < 0.05) and significantly influenced soil available nitrogen (p < 0.05), while other soil properties were less affected. Year effects were generally not significant, reflecting the stable microclimatic conditions under hoop-house cultivation. Mechanical property assessments indicated substantial declines in tensile load, tensile strength, and elongation at break after field use, especially for thinner films. Notably, Bio-0.006 and Bio-0.008 significantly improved fruit weight and soluble sugar content relative to PE (p < 0.05), leading to higher yields and better commercial quality. These results suggested that appropriately thin BDMs can satisfy agronomic requirements for watermelon under protected cultivation while minimizing plastic residues, offering a practical basis for optimizing biodegradable film thickness to balance mulching performance, productivity, and environmental sustainability.