Soil texture-driven modulation of poly-3-hydroxybutyrate (P3HB) biodegradation: Microbial shifts, and trade-offs between nutrient availability and lettuce growth

Poly-3-hydroxybutyrate (P3HB) is a promising alternative to persistent conventional plastics, capable of biodegrading within months. However, its microbial-driven degradation raises concerns about nutrient immobilization and impacts on plant growth. The biodegradation process occurs in multiple stages, during which shifts in the microbial community can alter soil properties and influence utilization of both intrinsic and polymer-derived organic matter. This study employs a novel approach to investigate how nutrient dynamics during the late stage of P3HB biodegradation affect lettuce (Lactuca sativa var. capitata cv. Brilliant) growth. Soil-to-sand mixtures (100_0, 80_20, 60_40, 40_60, 20_80, and 0_100 ratios) were spiked with P3HB, allowed to biodegrade for eight weeks, and then planted with sprouted lettuce seeds, which were cultivated for another eight weeks. P3HB addition inhibited plant growth and root development in all soil-sand mixtures. However, increasing the sand proportion enhanced plants' nitrogen content by 13-45 % compared to 100 % soil + P3HB. Depending on the sand-to-soil ratio, P3HB stimulated most enzymes involved in carbon, nitrogen and phosphorus acquisition. Basal and substrate-induced respirations were 9-209 % higher under P3HB addition compared to P3HB-free soil, likely due to an increase in the stabilized soil organic matter fraction. Residual P3HB analysis revealed that diluting soil with 20 % sand accelerated biodegradation, despite a decrease in bacterial abundance. In the 80_20 variant, the microbial community shifted toward higher fungal abundance, 19 % more than in 100_0 soil. While microbial proliferation was observed, it effect was outweighed by negative impacts on dry aboveground and root biomass. The highest P3HB biodegradation rate occurred in the 80_20 variant, underscoring soil texture as a critical factor in P3HB biodegradation. While microbial communities can degrade bioplastics, this process may compromise plant nutrient availability and hinder plant growth.